Many chemical reactions can be speeded up by substances called catalysts. A catalyst increases the rate of a chemical reaction, but can be recovered chemically unchanged at the end of the reaction. Living things are extraordinarily complex and a vast number of chemical reactions occur in all living cells.
The catalysts that enable chemical reactions to occur in living cells are called enymes. Without enzymes, the chemistry performed by living cells, called metabolism, would happen at too slow a rate for the organism to stay alive.
Characteristics of Enzymes
Enzymes are proteins, which themselves are long chains (polymers) of amino acids. These amino acids tangle themselves up into a ball shape, and hence most enzymes are globular proteins. The tangling is not random but is dependent on the nature of the amino acids that make up the protein, and how they interact with one another.
There are many hundreds of different enzymes, and each enzyme is specific for the substrate on which it acts. The substrate is the substance that is present at the beginning of the reaction. The substance which is made by the enzyme is called the product. For example, the digestive enzyme amylase, which is found in saliva, acts on the substrate starch to make glucose.
How do Enzymes Work?
The fact that each enzyme folds up into a special way dependent on the nature and order of amino acids in the chain, called its primary sequence, means that each enzyme has a particular 3-dimensional shape, called its tertiary structure.
Enzyme molecules are usually larger than the substrate molecules on which they act, and only a small part of the enzyme molecule actually comes into contact with the substrate. This region, known as the active site, is a small cleft which exposes certain amino acid residues which interact with the substrate molecules. The substrate molecules fit into the active site forming what is known as an enzyme-substrate complex. The chemical reaction is then able to take place, and the products are formed, which can then leave the active site; leaving the enzyme unaltered and free to interact with more substrate molecules.
This process is often referred to as the lock and key model.
Furthermore, each enzyme has a particular temperature range and pH (acidity or alkalinity range) at which it works best. If an enzyme is subjected to extremes of pH or excessive temperatures, irreversible changes can occur in the 3-dimensional structure of the enzyme which can also affect the active site. If this happens, the enzyme is said to be denatured. It has lost its powers of catalysis.
Naming of Enzymes
Many enzymes are named by adding the suffix 'ase' to the name of the substrate. Thus proteases (sometimes called proteinases) digest proteins, carbohydrases digest carbohydrates and lipases digest lipids (fats). The sugars sucrose and maltose are broken down by sucrase and maltase respectively.
Sometimes the names of the enzymes are less informative, for example the enzymes rennin and trypsin.
Putting Enzymes to Work
Production of Food
Human beings have unwittingly harnessed the properties of enzymes, produced by microbes, for thousands of years, in the production of wine, vinegar, bread, beer and cheese. These were the earliest applications of what later became the science of biotechnology. The discovery of fermentation to produce wines was attributed by the Greeks to Bacchus,the God of Wine.
Grapes usually contain extraneous yeasts on their surface, which are visible as a bloom. Hence, at its simplest, grapes are trampled to release the juice which is run into a suitable container. This is then kept at a steady warm temperature, whereupon the yeasts act upon the sugar in the grape juice, converting it to alcohol and carbon dioxide.
Yeasts are simple microscopic organisms belonging to the Fungi Kingdom. Their life depends on the energy from sugar, which they get from other living organisms. Yeasts contain enzymes which convert sugar (the substrate) into ethanol and carbon dioxide gas (the products). Indeed, the very name 'enzyme', coined by the German chemist, Wilhelm Kuhne in 1877, comes from the Greek en-zume, meaning 'in yeast'.
This process, which occurs anaerobically (i.e. in the absence of oxygen) is called fermentation.
If the wine is now left uncovered, a fungus, Mycoderma aceti, which flourishes in dilute alcoholic solutions, is able to enter. This produces enzymes which oxidise the ethanol to ethanoic acid, and the product is vinegar - which has its own uses.
Brief History of Brewing
The processes involved in the leavening of bread and beer making are essentially the same, also requiring yeast.
Brewing of beer is a craft which had its origins in ancient Mesopotamia from about 4000 BC. Here, bread was mashed, malted and fermented, and the resulting brew was flavoured with spices, dates or honey. About 1,000 years later, legend has it that Isis, the mother of the Gods, introduced it to Egypt, where bread and beer were staples of Ancient Egyptian life. At least six types of beer are thought to have been in daily use.
About 100 bread loaves have been discovered preserved in various Egyptian tombs, where there were also dozens of brewing jars. There are also tomb paintings showing bread being made. Some such loaves have been excavated from the Royal Bakery of Tutankhamun's father, the Pharaoh Akhenaten.
Furthermore, Queen Nefertiti, the step-mother of the boy-pharaoh, Tutankhamun is believed to have ordered the construction of a huge bakery-cum-brewery at Tell-el-Amarna, 320 km south of Cairo on the banks of the River Nile. Excavations of this site have been sponsored by the British brewers, Scottish and Newcastle, who also launched an ale, based on the authentic ancient recipe, called Tutankhamun's Tipple.
Beer making differs from wine making in that wine is made when yeast consumes the natural sugars found in fruits such as grapes. Beer is made when yeast consumes the sugar derived from grain, particularly barley. The naturally occurring starch found in grain must first be converted into sugar before yeast can consume it. Thus, beer making is a more complex art than wine making.
Making beer can be broken down into several discrete processes. The first stage, malting involves soaking the grain in water long enough to begin germination or sprouting. During malting, enzymes are produced which break down starches to sugars. Germination then has to be halted and this is done by kilning, a drying process, which involves heating in several stages.
The malt is then ground, or lightly crushed between rollers, and at this stage is known as 'grist', thus giving rise to the expression 'grist to the mill'. This process is known as 'cracking'.
The next stage, mashing, is the single most important operation in brewing, for it is here that the principal enzymatic changes occur. Mashing is the process of heating grains mixed with water at controlled temperatures for designated periods of time to activate various enzyme activities that convert starches to fermentable sugars. Converting starches to sugars is called saccharification. During mashing, the enzyme cytase dissolves the protective cellulose coating of the barley grains, giving access to the starch. The enzyme diastase then liquefies and converts starch to maltose and dextrins, which dissolve in the water to form a sweet, malt-flavoured liquor known as 'sweet wort'.
Once mashing is completed, the brewer must separate the wort from the spent grain husks. This is done by 'sparging' or rinsing the spent grains with hot water to extract as much sugar from the grains as possible. The grain husks act as a filter bed on the false bottom of the mash tun. After the wort is collected, it is boiled for one to two hours to prevent further enzyme activity. This is a critical step in the brewing process because it is at this step that hops, the aromatic flowers of hop vines, are added. Hops impart aroma, flavor, and bitterness to beer, which balances the sweetness of the wort.
When the boil is completed, the temperature of the wort is dropped to about 15°C, the fermentation temperature, following which yeast is added.
The main genus of yeast used by brewers is Saccharomyces, and the species used to make ales is a top fermenting strain called cerevisiae, while the species used to make lagers is a bottom fermenting strain called uvarum. The type of yeast used during fermentation determines whether a beer is an ale or a lager.
The yeast produces enzymes which convert malt sugar (maltose) to ethanol and carbon dioxide. This usually takes three to seven days and is referred to as primary fermentation.
The traditional malting process, as described above, is really just an expensive and inefficient way of manufacturing enzymes. So nowadays industrially produced enzymes such as amylases, glucanases and proteases are added to unmalted barley to produce the same products that malting would produce - by more controlled means. Use of industrial enzyme preparations in the brewing industry allows it to be more economic and have consistent quality.
In bread-making, baker's yeast is added to a mixture of flour and dough and it is the carbon dioxide that is produced during fermentation which causes the dough to rise. The alcohol is driven off during the baking process, so you won't get drunk by eating bread!
Flour, which gives bread its structure, is made by milling cereal grains such as wheat, barley, or rye. In this process, the grain seeds are crushed, releasing starch and proteins, some of which are enzymes. These enzymes modify the starch, protein and fibre of the flour when water is added.
Starch is a carbohydrate and starch molecules are polymers (chains) of simple sugars linked head to tail by chemical bonds. Enzymes, called carbohydrases, from the added yeast cells initially attack starch, breaking it down into a sugar called maltose. Other enzymes (maltases) convert maltose molecules into carbon dioxide and ethanol.
There are two proteins found in flour, gliadin and glutenin. When water is added to flour and kneaded, these proteins swell up like sponges and form a tough, sticky, elastic substance called gluten. During fermentation, carbon dioxide gas becomes entrapped in the gluten, causing the dough to rise.
Some maltose molecules are attacked by other enzymes present in the flour and are converted into alcohols, acids, and esters, all of which add to bread's flavour.
Once the dough has risen, following a couple of hours in a warm place, it is ready to be baked in the oven. There, heat causes pockets of gas in the dough to expand. Eventually the crust becomes a crusty brown and the bread is ready to be eaten.
In bakeries, the quality of the wheat flour varies as a consequence of natural variation, time of year or inconsistencies in milling. Nowadays, to improve consistency and efficiency, enzymes (such as xylanase, α-amylase, protease, glucose oxidase and lipase) are added.
Now, throughout the World, about 85 million tons of wheat flour is used every year to make bread, and a not inconsiderable quantity is thrown away due to its becoming stale. This occurs because the bread loses moisture, causing the bread to become hard within a few days. To slow this effect down an enzyme called maltogenic amylase, obtained from micro-organisms, is added to the flour. This alters the structure of the starch enabling it to retain moisture better and thus stay fresher for longer.
If one estimates that 10-15% of bread is thrown away, and if it could be kept fresh for another 5-7 days, then 2 million tons of flour per year could be saved; corresponding to 40% of the bread consumed in the USA.
The process of cheesemaking is an ancient craft that dates back thousands of years, which may even predate beer and bread making. It is a complicated process which is both an art and a science.
In cheese making, the basic raw material is milk, which may be obtained from a variety of animals, such as cows, sheep and goats. Thus, the specific cheesemaking process has to be modified in relation to the type of milk used.
The milk is warmed and a mixture of two enzymes (chymosin and pepsin)1 known as rennet, which is obtained from the fourth stomach of the milk-fed calf, is added. This coagulates the milk to form 'curds and whey'. The whey is a cloudy liquid which contains some protein and sugars (including 'milk sugar', lactose), while the curds are precipitated protein which is pressed and subsequently packed in various sized containers for maturing.
The need to coagulate milk has been well recognised since Roman times, and this is usually achieved by using an extract from calf stomach. However, the necessary enzymes are also found in certain plants, which are used in some European countries and the Far East. Since the use of calf stomach extract is a problem for vegetarians, many modern cheeses are produced using chymosin from fungi or bacteria, and sold as vegetarian cheese.
Use in Fruit and Vegetable Juice Technology
The use of enzymes as processing aids for fruit and vegetable production has become normal practice for many processes. Enzymes are used to maximize the production of clear or cloudy juice. This cloudiness arises from pectin which occurs in nearly all fruits. Pectins are structural polysaccharides (polymers of sugars) which occur mainly in the middle lamella and the primary cell wall of higher plants; and are largely responsible for the integrity and coherence of plant tissues. Although pectins generally make up less than 1% w/w of plant tissue, they cause cloudiness in extracted juices, and interfere with rapid filtration.
The addition of pectin degrading enzymes at the pressing stage increases the amount of juice produced and can reduce cloudiness. Two groups of such enzymes are distinguished, pectinesterases and pectin deploymerases. The latter group includes polygalacturonase, pectate lyases and pectin lyases. Pectinesterases appear in many fruit and vegetables, and are particularly abundant in citrus fruits and tomatoes. They are also produced by many fungi. They can also be obtained from commercial pectolytic enzyme preparations ('pectinase') which are commonly derived from the mould, Aspergillus niger. The application of enzymes in these processes is a combination of economic and cosmetic factors.
In contrast to the above, the desired flavour and colour of citrus juices especially orange depends on the insoluble, cloudy materials of the pressed juice. In these situations, the pectin component is manipulated requiring a balance between pectin methyl esterase, to promote cloudiness by increasing the pectin/calcium complex formation; and polygalacturonase, to break cloudiness by depolymerisation of the pectin.
Have you ever wondered how the liquid centre is placed into chocolates? Think enzymes! A chemist by the name of HS Paine invented liquid-centre chocolates in 1924 by exploiting the different solubilities of three sugars, table sugar (sucrose), glucose and fructose. Sucrose is a disaccharide containing the two simpler sugars: glucose and fructose. A mixture of equal parts glucose and fructose is very soluble in water, whereas sucrose is less soluble; and an equivalent amount of sucrose in water forms a paste-like solid. If the enzyme invertase (from yeast) is injected into the chocolate which contains sucrose paste, the sucrose is converted to glucose and fructose.
High Fructose Corn Syrup
Most of us are aware that, traditionally, sugar was obtained from sugar cane or sugar beet. However, most of the sugar sweeteners used in processed food nowadays, such as jams, ketchups and soft drinks, comes from sweetcorn. It is also in many so-called health foods.
The process for making the sweetener, known as 'high fructose corn syrup' (HFCS) from sweet corn was developed in the 1970s; the market growing rapidly from less than three million tons in 1980 to almost 8 million tons in 1995. During the late 1990s, use of sugar actually declined as it was eclipsed by HFCS. Today Americans, at least, consume more HFCS than sugar.
High-fructose corn syrup (HFCS) is produced by processing corn starch to yield glucose, and then processing the glucose to produce a high percentage of fructose. The process is somewhat complicated, involving the use of three different enzymes. Firstly, cornstarch is treated with α-amylase, of bacterial origin, to produce shorter chains of sugars called polysaccharides. Next, an enzyme called glucoamylase, obtained from the fungus Aspergillus niger, breaks the sugar chains down even further to yield the simple sugar glucose. The third enzyme, glucose-isomerase, is very expensive, and converts glucose to a mixture of about 42% fructose and 50%-52% glucose. Due to the expensive of the enzyme; glucose isomerase is immobilised on resin beads and the sugar mixture is then passed over it. This means it can be used multiple times until it eventually loses activity. (See 'Immobilised Enzyme Technology' below).
HFCS has the exact same sweetness and taste as an equal amount of sucrose from cane or beet sugar. Despite being a more complicated process than the manufacture of sugar, HFCS is actually cheaper. It is also very easy to transport, being piped into tanker trucks. Thus transport costs are lower and profits are higher for food producers.
It is worth knowing that two of the enzymes used, α-amylase and glucose-isomerase, are genetically modified to make them more thermostable. This involves exchanging specific amino acids in the primary sequence so that the enzyme is resistant to unfolding or denaturing. This allows the industry to use the enzymes at higher temperatures without loss of activity.
Other Culinary Applications
Most people will be familiar with the practice of placing a slice of pineapple onto gammon steak. This is because pineapple is a rich source of the bromelain group of enzymes, which are proteases, and hence break down protein. Thus pineapple is a natural meat tenderizer and a digestive aid.
Bromelain contains at least four cysteine proteases, with smaller amounts of acid phosphatase, peroxidase, amylase and cellulase. Bromelain has also been used successfully as a digestive enzyme following pancreatectomy, in cases of pancreas insufficiency, and in other intestinal disorders.
For the same reason, jelly made with either fresh pineapple or kiwi fruit does not set. This is because jelly consists of gelatin which is a protein, and insoluble. Bromelain which is in the fresh pineapple and kiwi fruit degrades the gelatin to form amino acids, which are soluble. Jelly made with canned pineapple does set because, during the canning process, pineapple is heated to a temperature high enough to denature the bromelain enzyme (a protein itself) making it functionless. Thus, the gelatin protein molecules remain intact and insoluble.
As mentioned above, the sugar that occurs in milk is lactose, which a disaccharide containing the monosaccharides galactose and glucose.
Though lactose intolerance may sound like a disorder, it is in fact perfectly natural. In most people the gene for lactase, the enzyme that digests lactose, is switched on at birth and switched off at the age of weaning. An estimated 75% of the World's population are intolerant to lactose in adulthood, due to not producing enough of the enzyme necessary (lactase or ß-galactosidase) to convert lactose to galactose and glucose. The lactose is left unabsorbed by the body, but the perfect conditions exist in the intestines for lactose to ferment, and this leads to the formation of intestinal gases. One particular gas, methane, is generally the cause for the pain and flatulence that is experienced.
In most Europeans, however, the infant condition persists, and the lactase gene remains active. It is thought that,with the domestication of cattle and goats in the Near East some 10,000 years ago, the ability to digest lactose throughout life could have conferred some nutritional advantage. Biologists speculate that a mutation that prolonged the gene's activity was suddenly favoured and spread throughout the population. The gene encoding lactase in humans is located on chromosome 1, and 70% of Westerners have a mutation so that the gene fails to switch off after infancy, thus conferring lactose tolerance. The other 30% of the population are 'lactose intolerant'.
Some of the symptoms may be similar to those of milk allergy but milk allergies can cause the body to react quicker, more often within a few minutes.
It is possible to inject lactase into cartons of UHT milk as it is packaged. It is also possible to purchase lactase which comes in the form of drops to add with milk, or as capsules to take before a meal. As enzymes are protein in nature, lactase would be denatured by stomach acid before it could take effect. Hence it is actually supplied as a pro-enzyme called Prolactazyme. The enzyme is then activated by partial digestion in the stomach, so that it has the opportunity to function in the small intestine.
Immobilised Enzyme TechnologyNow, the lactase enzyme is quite expensive, so nowadays the milk may be pre-treated with lactase before distribution. To achieve this on the industrial scale the enzyme is 'immobilised'. This means that the lactase is trapped onto an inert material, typically small resin beads. The beads are packed into a glass tube called a column. The milk is poured in at the top and allowed to trickle down the column slowly, enabling the enzyme to do its job. The chemical reaction takes place at the surface of the beads and the galactose and glucose, which are more easily digestible, are collected from the bottom of the column.
Other uses of immobilised enzyme technology include the production of whey syrup, which is used in confectionery in place of sweetened condensed milk. The whey is a by-product of cheese-making (see above), which contains lactose and protein. The lactose is hydrolysed to glucose and galactose; which taste sweeter and dissolve well, compared to the rather tasteless, poorly-soluble lactose.
Use in Medicine
Wine itself has great medical benefits. In past centuries they have been mixed with herbs and used to alleviate pain and other ailments. Wounds were often washed in wine to cleanse them and promote their healing. Even today, wine is sometimes recommended as a tonic after illness.
More specific applications of enzymes in modern medicine include:
Diabetes: In normal individuals the pancreas produces the hormone insulin, which is distributed via the blood stream to every cell of the body, thus enabling them to absorb glucose from the blood, required for energy. Some individuals, for a variety of reasons, are unable to produce sufficient insulin and must therefore inject themselves with prescribed insulin immediately after every meal. It would be useful for such individuals (diabetics) to measure their blood sugar level throughout the day in order to regulate their use of insulin. Fortunately, the blood sugar level is linked to the concentration of glucose in the urine and home-based tests are now available to enable them to measure this.
One such test (Clinistix) relies upon a chemical reaction that produces a colour change on a test strip. Generally, the test strip is placed in a urine sample. The resulting colour change is matched against a colour chart provided by the manufacturer which shows the different colours produced by different levels of glucose. The test strip contains a chemical indicator called toluidine and the enzyme glucose oxidase. Glucose oxidase converts the glucose in urine to gluconic acid and hydrogen peroxide. The interaction of the hydrogen peroxide with the toluidine causes a change in colour.
Enzyme Deficiency Diseases: About 2,000 different enzymes are required for the human body to function, and these need to be coded for by DNA. A variety of metabolic diseases are now known to be caused by deficiencies or malfunctions of enzymes, due originally to gene mutation. Albinism, for example, is often caused by the absence of tyrosinase, an enzyme essential for the production of cellular pigments. The hereditary lack of phenylalanine hydroxylase results in the disease phenylketonuria (PKU) which, if untreated, leads to severe mental retardation in children. Although PKU is usually managed by dietary modifications, intravenous 'Enzyme Replacement Therapy' can sometimes be employed in some enzyme deficiency diseases.
One such example is Gaucher's Disease Type 1, where the body lacks sufficient levels of the enzyme glucocerebrosidase. This is needed for breakdown of fatty materials, or lipids. This deficiency causes lipids to accumulate, swelling the spleen and liver, crowding out the marrow in the bones, and triggering anaemia and low blood platelet counts. Such patients often suffer from fatigue, grossly distended abdomens, joint and bone pain, repeated bone fractures, and increased bruising and bleeding. This can be treated using intravenous enzyme replacement therapy with a modified version of the enzyme, known generically as alglucerase.
Development of medical applications for enzymes have been at least as extensive as those for industrial applications and the variety of enzymes and their potential therapeutic applications (requires pdf reader) are considerable; reflecting the magnitude of the potential rewards. For example, pancreatic enzymes have been in use since the nineteenth century for the treatment of digestive disorders.
Heart Attacks: The enzyme streptokinase is administered intravenously to patients as soon as possible after the onset of a heart attack to dissolve clots in the arteries of the heart wall. This minimises the amount of damage to the heart muscle. Streptokinase belongs to a group of drugs known medically as 'fibrinolytics', or colloquially as 'clotbusters'. It works by stimulating extra production of a naturally-produced protease called plasmin. Plasmin is produced in the blood to break down fibrin, which is the major constituent of blood clots, therefore dissolving clots once they have fullfilled their purpose of stopping bleeding.
Acute Childhood Leukaemia: A major potential therapeutic application of enzymes is in the treatment of cancer. The enzyme, asparaginase, extracted from bacteria, has proved to be particularly useful for the treatment of acute lymphocytic leukaemia in children, in whom it is administered intravenously. Its action depends upon the fact that tumour cells are deficient in an enzyme called aspartate-ammonia ligase. This restricts their ability to synthesise the normally non-essential amino acid L-asparagine, but which is essential in leukaemic cells. Therefore, they are forced to extract it from body fluids. The action of the asparaginase does not affect the functioning of normal cells which are able to synthesise enough for their own requirements, but reduces the free circulating concentration, thus starving the leukaemic cells. A 60% incidence of complete remission has been reported in a study of almost 6,000 cases of acute lymphocytic leukaemia.
Cystic Fibrosis (CF) This is a very important example of a therapeutic enzyme because it illustrates the enormous potential rewards from biotechnology. In this treatment the enzyme, deoxyribonuclease', or DNA'ase for short hydrolyses, or breaks down, extracellular DNA. Patients with CF generally suffer badly from bacterial infections in the lungs which is associated with a heavy build-up of thick mucus. Doctors generally prescribe antibiotics, but CF patients also require daily percussion therapy, ie physical pounding on their chests for up to one hour. It was long assumed that the mucous was a response to bacteria, but studies have shown that there was a huge amount of DNA present in the mucous. This DNA arises from dead white blood cells and bacterial cells. Being long, thin and highly electrically charged, this DNA serves to cross link the mucous thus changing it from a fluid gel to a semi-solid custard. It is estimated there are 30,000 CF patients in the USA and 20,000 in Europe, and the enzyme costs £6,000 per year for 2.5 mg of enzyme per day.
Using Enzyme Inhibitors. It is possible to increase the efficacy of the penicillin antibiotics by inhibiting the action of a bacterial enzyme. Penicillins are a class of antibiotics which inhibit synthesis of the bacterial cell wall, which is polysaccharide in nature (ie consists of a polymer of sugar molecules). The bacteria can develop resistance to penicillins by producing enzymes called beta-lactamases, which break down penicillins. It is possible to block the active sites of beta lactamase using a broad spectrum antibiotic known as Augmentin marketed by GSK.Diagnosis of Disease
The presence of enzymes where they should not be present can also help to diagnose disease. For example, when the liver is diseased or damaged, enzymes such as alanine aminotransferase leak into the bloodstream. Testing the blood for these enzymes can confirm liver damage.
The chemical synthesis of complex drugs is often difficult and companies often turn to enzymes to perform chemical conversions. Enzymes are particularly useful when it comes to small-molecule pharmaceutical chemicals. This is because they are stereo-specific and are thus able to make single-isomer (chiral) compounds, whereas ordinary chemical methods normally yield mixtures of stereo-isomers.
Many organic molecules exist in left-handed and right-handed forms. This property, known as chirality, is a consequence of the way the four bonds from a carbon atom arrange themselves in space. They point to the four corners of a tetrahedron - a pyramid with a triangular base. If the four chemical groups attached to the carbon atom are different, then there are two possible arrangements or configurations. One configuration is the mirror image of the other, like the relationship between one's left and right hand. The images are said to be 'non super-imposable'. The two forms, which are stereoisomers, are known as enantiomers.
Enantiomers have virtually identical physical properties and identical chemical properties - except towards other chiral compounds like enzymes found in the body. The only way in which their physical properties differ is in the way they rotate the plane of polarised light (and hence are known also as optical isomers). This optical activity is observed with a polarimeter. If the rotation is to the right, the compound is said to be dextrorotatory (designated d or +), whilst if the rotation is to the left, it is known as laevorotatory (designated l or -).
When a chiral compound is synthesised in the laboratory, an equal mixture of the right-handed (D, dextro) and the left-handed (L, laevo) molecules is normally produced. Such a mixture is known as a racemic mixture. They show no optical activity because the equal and opposite effects of light rotation cancel each other out.
Living organisms, on the other hand, synthesise chiral molecules predominantly with a specific handedness. Virtually all proteins are polymers of L-amino acids, for example, and the cell machinery which produces proteins is itself composed of L-amino acids and D-sugars.
The fact that ordinary chemical synthesis normally results in a racemic mixture had tragic consequences in the 1960s with the drug, thalidomide. This was taken by pregnant women as a sedative and to prevent morning sickness, but many subsequently gave birth to children deformed by phocomelia. Later research showed that the (+) isomer had the desired effect whilst the (-) isomer had a teratogenic effect on rat embryos, giving the same birth defect as those of thalidomide children. Nowadays optically pure thalidomide is used in the treatment of leprosy, Behcet's syndrome, AIDS and TB.
Today, although there are ways of separating the enantiomers in a racemic mixture, and there are also so-called 'chiral synthesis' reagents, chemists are turning increasingly to nature's chiral catalysts - enzymes - to achieve the same aim. These are being used in the synthesis of single enantiomer versions of drugs from antidepresssants to anti-inflammatories; and agrochemicals such as pesticides.
Drug manufacturers may either use the purified enzymes or enzyme-containing microrganisms to biocatalyse a given chemical reaction. A typical example is the enzyme-based production of (-)-lactam, an intermediate for carboxylic nucleosides such as the AIDS drug abacavir sulfate, produced by ChiroTech.
Chirality isn't always the driver, however. Biocatalysis can be used to carry out conversions that would otherwise require difficult or multiple synthetic steps. For example, the vitamin nicotinamide is produced by enzymatic hydrolysis of 3-cyanopyridine.
Use in Washing Powders
The main stains that need to be removed from clothing are organic in origin; for example, proteins, fats and carbohydrates which are all present in foodstuffs. Hence biological washing powders may contain proteases, lipases and carbohydrases. The carbohydrases may include amylases, which are effective on removing starchy food deposits. Some powders contain cellulase to brighten colours and soften fabrics.
Proteases and amylases are also effective in dishwasher detergents, to remove food particles. These detergents are environmentally friendly with fewer bleaching agents and phosphates, allowing the enzymes to work more effectively whilst having minimal effects on public and environmental health. However, as biological washing powders contain proteases and the connective tissue in skin is made up largely of the proteins, collagen and elastin, biological washing powders can cause 'Irritant Contact Dermatitis'. It is prudent, therefore, not to allow prolonged exposure of the skin to such products, and to ensure that all garments are adequately rinsed.
Use in the Textile Industry
Enzymes are used in the leather and the textile industries in finishing processes. Proteases help in the de-hairing of the animal hides and lipases are used for de-greasing. The correct application of a cellulase enzyme can give a smoother, glossier, brighter fabric to cellulose fibres like cotton. This technique is known as bio-polishing. In the denim industry, cloth was traditionally stonewashed with pumice stones to fade the fabric. This depends upon physical damage to the fibres in denim to allow the dye molecules to escape. A small application of cellulase minimises damage to the garments and also to machinery. This technique is known as bio-stoning and can ensure greater fading without high abrasive damage to fabric and accessories (buttons, rivets).
Enzymes have been used, albeit unwittingly, for thousands of years, in the production of foodstuffs such as bread, beer and cheese. These days, commercially purified, and sometimes immobilised, ezymes are used by industry and medicine because of their catalytic abilities, which ensure that they can be recovered chemically unchanged at the end of the reaction and thus used again. Enzymes are useful because they can be used in minimal quantities and at relatively low temperatures, and thus keep costs down. In medicine, they are useful because of their unique specificity, thus avoiding side effects when used on a patient. They are invaluable in synthetic organic chemistry, for example in the synthesis of drug molecules, because they are stereo-specific thus enabling the cheap and convenient production of chiral molecules.