Abstract
Enzymes are biological catalysts whose function is to increase the rate of metabolic reactions.
Enzymes are biological catalysts whose function is to increase the rate of metabolic reactions.
What is a catalyst?
A catalyst is a substance that increases the rate of a chemical reaction without being itself chemically altered. As the catalyst is not consumed in the reaction, it can be involved in repeated chemical reactions – only relatively small numbers of catalyst molecules are required.
What are the main features of an enzyme?
Enzymes are complex, three-dimensional proteins that have three important features:
Catalysis. Enzymes act as catalysts for biological reactions.
Specificity. Their complex, three-dimensional structure results in a highly specific binding site – the active site – for the reacting molecules or substrates. The active site can even distinguish between different stereoisomers of the same molecule.
Regulation. Many of the reactions in biochemical pathways (e.g. the glycolytic pathway) are very slow in the absence of enzymes. Therefore, the rate of a biochemical pathway can be controlled by regulating the activity of the enzymes along its path, particularly the enzyme controlling the rate-limiting step, which in the case of glycolysis is phosphofructokinase.
How does an enzyme work?
Enzymes work by binding substrates in a particular orientation, bringing them into the optimal position to react together. This lowers the activation energy for the chemical reaction, which dramatically increases the rate of reaction. The three-dimensional shape of the active site is of crucial importance. If the shape of the active site is altered (e.g. by increased temperature or pH), the function of the enzyme may be impaired and the chemical reaction slowed.
As an example, the reaction between CO2 and water giving carbonic acid (H2CO3) is very slow:
However, addition of the enzyme carbonic anhydrase (CA), which contains a zinc atom at its active site, to the mixture of CO2 and water increases the speed of the reaction considerably. First, water binds to the zinc atom, then a neighbouring histidine residue removes an H+ ion from the water, leaving the highly active OH‾ ion attached to zinc (Figure 5.1). Finally, there is a pocket within the active site that fits the CO2 molecule perfectly: with CO2 and OH‾ in close proximity, the chemical reaction takes place quickly. Once CO2 and water have reacted, the resulting H2CO3 diffuses out of the enzyme, leaving it unchanged chemically; that is, the enzyme acts as a catalyst.
The same enzyme can also catalyse the reverse reaction. This is indeed the case for CA, which catalyses
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