Enzymes play a large part in the biological world and this has been transferred into industrial methods to save both time and money. Harnessing the power of enzymes has changed the modern world, and has made it feasible to mass-produce many products, which beforehand would have taken weeks, even months to produce. The quality of products has also been raised as a result. Possibly the largest industry today that uses enzymes, is the food industry with many different functions across the board.
Pectin Esterase is an enzyme designed to modify pectin molecules in a precise manner, in an attempt to maintain the shape and texture of food, particularly in fruit. Plant cells have a layer between them made predominantly from pectin – see figure 1. It is this layer that effectively holds the cells together in a rigid structure, or tissue. If this were to be completely removed, the tissue would fall apart, which is what happens when fruits go soft. 1
As fruits grow, they become softer, riper, due to the effects of enzymes such as pectinase, slowly breaking down this pectin layer. However it is not entirely known exactly which enzymes are responsible for this development. It is hoped that one day it may be possible to monitor this activity and hence pick fruits just as this occurs.
For example, in a punnet of strawberries, many of them are often bruised and unsightly by the time they reach the supermarket from the warehouse, even before they go on sale. This artificially created enzyme will try to sustain the fruit’s shape and structure, thus increasing the number of ‘good’ strawberries in the product sold. By stripping the methyl groups from pectin molecules in the fruit, links and bonds can be formed between them, effectively ‘gelling’ the pectin together, strengthening the fruit overall. As a result, the fruit is stronger, and will withstand the knocks and blows more effectively, leaving the customer with a more attractive product.
Another enzyme that makes foodstuffs more appealing is lactase.2 This has many uses, one of which is to treat ice cream. If the milk used to make the ice cream is lactose treated, or the ice cream mix itself is hydrolysed with lactase, then lactose crystals are not allowed to form. Lactose crystals give the ice cream a sandy appearance and can deteriorate the taste – by adding lactase this problem is eliminated. It also makes the ice cream easier to ‘scoop’.
Lactase can also be used in the treatment of milk, for those who are intolerant to lactose. Domestic cats must also be fed lactose-reduced milk for health purposes. When producing condensed milk, using lactase stops lactose crystallisation and so the milk is less likely to thicken. Lactase breaks down lactose, a disaccharide, into monosaccharides – glucose and galactose. These products are much sweeter than lactose, and therefore can be used in lessened quantities, saving money in the long run. Glucose Isomerase does a similar thing in the respect that it converts D-glucose into D-fructose, which is a sweeter alternative. This is especially used in the confectionary and soft drink industries.
Invertase is another enzyme that has changed the industry of confectionary. This is the enzyme that allows chocolates to have a fondant or soft centre to them, like after dinner mints, liqueur chocolates and Easter eggs like Cadbury’s “Crème Eggs”. The centre of the chocolate or sweet is made to a fudge-type consistency using sucrose, and adding invertase before coating in chocolate and being left to cool. During the standing time after packaging, the invertase becomes active and turns the sucrose into monosaccharides, glucose and fructose. It is this reaction which gives the chocolate a liquid or fondant centre. An American chemist, H.S Paine, first suggested this in 1924.
Beer brewing is another major industry that relies on the use of enzymes, this time a living one; yeast, in its production. Essentially, yeast acts on barley, maize, hops and other such plant products to produce ethanol – alcohol. They do this by converting sugars into alcohol and carbon dioxide, but the sugars in these plant products are very complex, in the form of starch, and cannot be made use of efficiently. Therefore, the barley is left to germinate a little, which releases enzymes that catalyse the conversion of complex carbohydrates into simple sugars. The grains are then dried and roasted, killing off the germinating sprouts.
The grain is cracked, sprouts removed and crushed to help convert more sugars and kill bacteria in the grain. More sugars are created from the complex carbohydrates in the grain by applying heat. Chemical bonds break and the resulting pieces are smaller, simpler sugars, which can be easily fermented by the yeast. Hops and flavourings are added and boiling helps kill any remaining bacteria that may compete with the yeast. This mixture, wort, is then cooled to the optimum temperature for the specific variety of yeast, which is then left to ferment, producing alcohol.
Unfortunately this is quite an expensive process, and difficult to control effectively, so industrial enzymes such as amylase, glucanase and protease can be added for more efficient and controlled alcohol production. When this is being filtered, viscous polysaccharides can slow down the process. By pre-treating with xylanase or glucanase, these polysaccharides are broken down, speeding up filtration and making the procedure much more economical and cost-effective.
Fruit juices also use enzymes in their manufacture, let us use apple juice as an example for non-citrus fruits. In unripe fruit the pectin that makes up a lot of the cell wall, is insoluble, but as the fruit ripens, some of the pectin chains are broken down and thus the tissue softens. As a result, this becomes soluble and when the apple is crushed some of this pectin is taken, along with the juice. It is this that makes the juice have a hazy appearance. Once all the apples have been crushed, they are incubated, along with pectinase, which breaks up the soluble pectin, allowing the juice to flow more freely. However, this enzyme also breaks down insoluble pectin, which can thicken the juice again.
If a cloudy product is wanted, then the apples are pressed and pasteurised (heated to a high temperature quickly) immediately to denature the enzymes. This is then centrifuged to removed large pieces, whereas small pieces are left in suspension – giving the cloudy appearance. If a clear juice is required, these small particles must be removed. Some soluble pectin is still left in the juice, which means that filtering would not work. It must be treated with pectinase again to remove this. Starch may also have caused the juice to be cloudy, and this is also removed used an enzyme. Amylase is often added at the same time as pectinase to solve this problem. This is then centrifuged to remove all the debris and the enzymes, and then a clear juice is produced.3
Citrus fruits however must be made into juice using a different method. The juice is much harder to extract in citrus fruits, because the pectin holds the liquid in. For that reason, pectinase must be used to remove the pectin itself and allow the juice to come free from the fruit pulp. It is important that some of the insoluble pectin remains in the juice to give it a cloudy appearance. It is possible to use artificial agents to produce the same effect, but these are illegal in many countries. Instead pectinase is used. This is ironic as one would think that this enzyme would break down the pectin, but we must remember that pectin is not one single substance, but a group of polysaccharides.
If the juice is not cloudy enough, more pectin must be added. This is found in abundance in the white layer, pith, beneath the skin of most citrus fruits. By grinding this, mixing with water and boiling, the tissue is softened and after cooling, pectinase is added. This breaks down the peel tissue forming a cloudy liquid containing cellulose, pectins, and cell organelles. By pasteurising to denature the enzymes, centrifuging to remove the cell organelles and concentrating, the process is complete and this can be added to the existing juice.
Although the food industry is probably the largest user of enzymes, the best-known application of them is in detergents and washing powders.4 They have been used since the 1960’s in this way. The main enzyme in laundry detergents is protease, which breaks down proteins such as grass stains, many food products, blood and human sweat. Recently however, this has been improved, to include a mixture of enzymes, incorporating lipases to acts on stains from fat-based products like oils, grease, makeup, and amylases, which act on starchy food types. Colour-enhancing washing powders have also been introduced which contain cellulases, which are there to remove cellulose fibrils, which dull the colour of the clothing over time.
Many companies have developed a pre-soaking detergent and spot application for tougher stains. Enzymes contained in these are much stronger than those in the normal detergent, and aim to loosen the soiling before the main wash. This would save money on detergent costs, and also save on energy, as clothes could be washed at a lower temperature. The same principle can be applied to dishwashing detergent, which also contains protease and amylase to remove food particles from crockery. These are also more environmentally friendly as they contain less bleach and phosphates.5
Although the uses of enzymes are largely beneficial, there are some issues that are still under discussion. For example, large scale productions of such enzymes is proving costly and is also having an effect on the environment. Many would argue that nature had not intended for humans to interfere and change its path and some would even call it “playing God”.
Many investigations are currently being carried out to explore the possibility of multi functioning enzymes. For example Limonoid Glucosides are known to stop citrus juice turning bitter, but recent studies have shown that they also inhibit the growth of tumour cells, including human cancer cells. Enzymes are an extremely important part of the biological world, and the extent of their power is only beginning to be realised. Our increasing knowledge of DNA technology also means that we could have the ability to construct enzymes from amino acids for specific purposes, once we know the coding required. This would create endless possibilities and could be particularly groundbreaking in the medical field.
The genes for several potentially useful enzymes have been identified, but a lot of work must be done before these are approved and accepted for general usage. This must be thought through very carefully before being put into practice. On the whole, enzymes contribute extensively to the industry so that demands on both quantity and quality can be met.
Enzyme kinetics is the study of factors that determine the speed of enzyme-catalysed reactions. It utilizes some mathematical equations that can be confusing to students when they first encounter them. However, the theory of kinetics is both logical and simple, and it is essential to develop an understanding of this subject in order to be able to appreciate the role of enzymes both in metabolism and in biotechnology.
Assays (measurements) of enzyme activity can be performed in either a discontinuous or continuous fashion. Discontinuous methods involve mixing the substrate and enzyme together and measuring the product formed after a set period of time, so these methods are generally easy and quick to perform. In general we would use such discontinuous assays when we know little about the system (and are making preliminary investigations), or alternatively when we know a great deal about the system and are certain that the time interval we are choosing is appropriate.
In continuous enzyme assays we would generally study the rate of an enzyme-catalysed reaction by mixing the enzyme with the substrate and continuously measuring the appearance of product over time. Of course we could equally well measure the rate of the reaction by measuring the disappearance of substrate over time. Apart from the actual direction (one increasing and one decreasing), the two values would be identical. In enzyme kinetics experiments, for convenience we very often use an artificial substrate called a chromogen that yields a brightly coloured product, making the reaction easy to follow using a colorimeter or a spectrophotometer. However, we could in fact use any available analytical equipment that has the capacity to measure the concentration of either the product or the substrate.
In almost all cases we would also add a buffer solution to the mixture. As we shall see, enzyme activity is strongly influenced by pH, so it is important to set the pH at a specific value and keep it constant throughout the experiment.
Our first enzyme kinetics experiment may therefore involve mixing a substrate solution (chromogen) with a buffer solution and adding the enzyme. This mixture would then be placed in a spectrophotometer and the appearance of the coloured product would be measured. This would enable us to follow a rapid reaction which, after a few seconds or minutes, might start to slow down, as shown in Figure 4.
A common reason for this slowing down of the speed (rate) of the reaction is that the substrate within the mixture is being used up and thus becoming limiting. Alternatively, it may be that the enzyme is unstable and is denaturing over the course of the experiment, or it could be that the pH of the mixture is changing, as many reactions either consume or release protons. For these reasons, when we are asked to specify the rate of a reaction we do so early on, as soon as the enzyme has been added, and when none of the above-mentioned limitations apply. We refer to this initial rapid rate as the initial velocity (v0). Measurement of the reaction rate at this early stage is also quite straightforward, as the rate is effectively linear, so we can simply draw a straight line and measure the gradient (by dividing the concentration change by the time interval) in order to evaluate the reaction rate over this period.
We may now perform a range of similar enzyme assays to evaluate how the initial velocity changes when the substrate or enzyme concentration is altered, or when the pH is changed. These studies will help us to characterize the properties of the enzyme under study.
The relationship between enzyme concentration and the rate of the reaction is usually a simple one. If we repeat the experiment just described, but add 10% more enzyme, the reaction will be 10% faster, and if we double the enzyme concentration the reaction will proceed twice as fast. Thus there is a simple linear relationship between the reaction rate and the amount of enzyme available to catalyse the reaction (Figure 5).
This relationship applies both to enzymes in vivo and to those used in biotechnological applications, where regulation of the amount of enzyme present may control reaction rates.
When we perform a series of enzyme assays using the same enzyme concentration, but with a range of different substrate concentrations, a slightly more complex relationship emerges, as shown in Figure 6. Initially, when the substrate concentration is increased, the rate of reaction increases considerably. However, as the substrate concentration is increased further the effects on the reaction rate start to decline, until a stage is reached where increasing the substrate concentration has little further effect on the reaction rate. At this point the enzyme is considered to be coming close to saturation with substrate, and demonstrating its maximal velocity (Vmax). Note that this maximal velocity is in fact a theoretical limit that will not be truly achieved in any experiment, although we might come very close to it.
The relationship described here is a fairly common one, which a mathematician would immediately identify as a rectangular hyperbola. The equation that describes such a relationship is as follows:
The two constants a and b thus allow us to describe this hyperbolic relationship, just as with a linear relationship (y = mx + c), which can be expressed by the two constants m (the slope) and c (the intercept).
We have in fact already defined the constant a — it is Vmax. The constant b is a little more complex, as it is the value on the x-axis that gives half of the maximal value of y. In enzymology we refer to this as the Michaelis constant (Km), which is defined as the substrate concentration that gives half-maximal velocity.
Our final equation, usually called the Michaelis–Menten equation, therefore becomes:
In 1913, Leonor Michaelis and Maud Menten first showed that it was in fact possible to derive this equation mathematically from first principles, with some simple assumptions about the way in which an enzyme reacts with a substrate to form a product. Central to their derivation is the concept that the reaction takes place via the formation of an ES complex which, once formed, can either dissociate (productively) to release product, or else dissociate in the reverse direction without any formation of product. Thus the reaction can be represented as follows, with k1, k−1 and k2 being the rate constants of the three individual reaction steps:
The Michaelis–Menten derivation requires two important assumptions. The first assumption is that we are considering the initial velocity of the reaction (v0), when the product concentration will be negligibly small (i.e. [S] ≫ [P]), such that we can ignore the possibility of any product reverting to substrate. The second assumption is that the concentration of substrate greatly exceeds the concentration of enzyme (i.e. [S]≫[E]).
The derivation begins with an equation for the expression of the initial rate, the rate of formation of product, as the rate at which the ES complex dissociates to form product. This is based upon the rate constant k2 and the concentration of the ES complex, as follows: 1
Since ES is an intermediate, its concentration is unknown, but we can express it in terms of known values. In a steady-state approximation we can assume that although the concentration of substrate and product changes, the concentration of the ES complex itself remains constant. The rate of formation of the ES complex and the rate of its breakdown must therefore balance, where: and
Hence, at steady state:
This equation can be rearranged to yield [ES] as follows: 2
The Michaelis constant Km can be defined as follows:
Equation 2 may thus be simplified to: 3
Since the concentration of substrate greatly exceeds the concentration of enzyme (i.e. [S] ≫ [E]), the concentration of uncombined substrate [S] is almost equal to the total concentration of substrate. The concentration of uncombined enzyme [E] is equal to the total enzyme concentration [E]T minus that combined with substrate [ES]. Introducing these terms to Equation 3 and solving for ES gives us the following: 4
We can then introduce this term into Equation 1 to give: 5
The term k2[E]T in fact represents Vmax, the maximal velocity. Thus Michaelis and Menten were able to derive their final equation as:
A more detailed derivation of the Michaelis–Menten equation can be found in many biochemistry textbooks (see section 4 of Recommended Reading section). There are also some very helpful web-based tutorials available on the subject.
Michaelis constants have been determined for many commonly used enzymes, and are typically in the lower millimolar range (Table 5).
It should be noted that enzymes which catalyse the same reaction, but which are derived from different organisms, can have widely differing Km values. Furthermore, an enzyme with multiple substrates can have quite different Km values for each substrate.
A low Km value indicates that the enzyme requires only a small amount of substrate in order to become saturated. Therefore the maximum velocity is reached at relatively low substrate concentrations. A high Km value indicates the need for high substrate concentrations in order to achieve maximum reaction velocity. Thus we generally refer to Km as a measure of the affinity of the enzyme for its substrate—in fact it is an inverse measure, where a high Km indicates a low affinity, and vice versa.
The Km value tells us several important things about a particular enzyme.
An enzyme with a low Km value relative to the physiological concentration of substrate will probably always be saturated with substrate, and will therefore act at a constant rate, regardless of variations in the concentration of substrate within the physiological range.
An enzyme with a high Km value relative to the physiological concentration of substrate will not be saturated with substrate, and its activity will therefore vary according to the concentration of substrate, so the rate of formation of product will depend on the availability of substrate.
If an enzyme acts on several substrates, the substrate with the lowest Km value is frequently assumed to be that enzyme's ‘natural’ substrate, although this may not be true in all cases.
If two enzymes (with similar Vmax) in different metabolic pathways compete for the same substrate, then if we know the Km values for the two enzymes we can predict the relative activity of the two pathways. Essentially the pathway that has the enzyme with the lower Km value is likely to be the ‘preferred pathway’, and more substrate will flow through that pathway under most conditions. For example, phosphofructokinase (PFK) is the enzyme that catalyses the first committed step in the glycolytic pathway, which generates energy in the form of ATP for the cell, whereas glucose-1-phosphate uridylyltransferase (GUT) is an enzyme early in the pathway leading to the synthesis of glycogen (an energy storage molecule). Both enzymes use hexose monophosphates as substrates, but the Km