Revision:Enzymes

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Enzymes - A-level Biology Revision Notes

Introduction

There are number of basic facts that you need to know about enzymes before you can start to understand the more advanced principles of enzyme activity and management. Most of these should have been covered at GCSE level.

• Enzymes are Globular proteins - This means that they are tertiary proteins and have a highly complicated folded structure. They are also mainly soluble. Globular proteins like all proteins are made of amino-acid subunits joined together by hydrogen bonds, disuphide bridges and ionic bonds.

• Enzymes are Biological catalysts - This means that they speed up chemical reactions without being used up in the process meaning that only a small amount of them is needed. They work by reducing the activation needed for the reaction to take place.

• Enzymes are Specific - This means that each enzyme will only catalyse one or possibly two reactions. This is because they have and activation site which is highly specific to the shape of the reactants. An enzyme can either join two reactants together or break them apart. An example of a reactions where enzymes are used to catalyze is the conversion of lactose to glucose and galactose by lactase.

How do Enzymes work?

The theory of enzyme activity is known as the lock and key theory. There is an area on the surface of the enzyme known as the active site, the shape of which is highly specific to the shape of the substrate. This give the substrate a surface on which to react.

Image:Lock and key.png

What Factors can affect enzyme activity?

There are four main factors which influence the activity of enzymes. This makes a common exam question so it is important that you know this.

Temperature

Temperature has a great impact on the activity of enzymes. Initially an increase in temperature is linked to an increase in activity of the enzyme. This is due to an increase in kinetic energy which leads to an increase in the frequency of successful collisions between the enzyme and substrate. However this is only true to an extent if the temperature is increased beyond the optimal temperature of the enzyme (which in most cases is 40-45°C), the bonds will start to break and the shape of the active site will change. This is known as denaturing.

pH

The pH of the solution which contains the enzyme has an impact on the activity of the enzyme. This is because the pH affects the ionisation state of the R group in the amino acid. So this also also effects the bonding within the enzyme and the shape of the active site. Extremes of pH will denature the enzyme and stop is working as a catalyst. As with temperature enzymes will also have an active pH, this varies depending on the site that the enzyme works in for example pepsin works in the stomach and has an optimal pH of 2 while amylase which is found in the saliva has an optimal pH of 7.8.

Enzyme concentration (and its effects on enzyme activity)

As enzymes are biological catalysts ( i.e. they can be used again and again without being used up.), they work efficiently at very low concentrations. As long as the temperature and the pH are suitable for the reaction, (and provided there are excess substrate molecules) the rate of reaction is directly proportional to the enzyme concentration.

If the amount of substrate molecules is restricted, substrate concentration will be the limiting factor in the reaction. Therefore, increasing enzyme concentration will not increase the rate of reaction and the rate will remain at a maximum constant.

Substrate concentration (and its effects on substrate concentration)

For a fixed amount of enzyme molecules, the rate of reaction is directly proportional to substrate concentration. If substrate concentration is low then the active sites of the enzymes are not used up.

As substrate concentration increases, more active sites come into use. Eventually, all active sites are being used almost simultaneously.

At this stage, increasing the substrate concentration will not increase the rate of reaction. Enzyme concentration has become the limiting factor in the reaction.

Enzyme inhibition

An inhibitor is a substance that reduces the activity of enzymes. There are two main sorts of enzymes inhibitors. Those which are directed at the active site and compete with the substrate are known as active site directed inhibitors. The binding can be irreversible as is the case of heave metal ion inhibitors or reversible for example malonate in the respiratory pathway. These types of inhibition have the potential to be reversed by increasing the amount of substrate. There second sort of enzyme inhibition is known as non-active site directed inhibition this is when the inhibitor binds to another part of the enzyme and changes the shape of the active site so that they substrate cannot bind. An example of this is cyanide.

Industrial Use of Enzymes

Pectinases

Pectinase is used in the extraction of fruit juices. The pectinase is add once the fruit has been crushed and release the juice by breaking down the pectin which holds the plant cell walls together. The use of pectin in the juice production process also makes the juice less cloudy. The type of pentinase used varies between fruit, if the fruit has a low pH a pectinase with a low optimum pH will be used etc.

Proteases

Proteases are used in biological washing powders to help break down the dirt on the clothes which can be made of food stains, blood or sweat. The proteases work by hydrolysing the protein to small peptides which then dissolve in the water and are removed. The proteases used have been taken from thermophillic Bacteria and so have a high optimum temperature compatible with the high temperatures used in clothes washing.

Lactases

Lactase is often used in the production of food products without lactose in them for people who are lactose intolerant. The lactase enzyme is suspended in alginate beads in a process also known as immobilisation and the milk is passed through it causing the lactase to be broken down into glucose and galactose.

Comments

Created from class notes by randdom.