Even the simplest of organisms have hundreds of enzymes in every living cell, catalyzing reactions that are crucial for life. A classification systems exists that categorizes all the known enzymes based on the general class of reaction that they catalyse.
There are six groups, described below.
This pie chart shows the distribution of all the known enzymes.
It is clear that the most populous group is the hydrolases, followed by oxidoreductases and transferases. The proportion of enzymes in the other groups is significantly less.
What are oxidoreductases?
Oxidoreductases catalyse oxidation or reduction reactions, where electrons are transferred from one molecule (the reductant) to another molecule (the oxidant).
This can be shown as:
A- + B --> A + B-
Where A= the reductant and B= the oxidant and an electron has transferred from A to B.
This process often requires co-factors such as NAD(P)H.
They are very important enzymes, which are vital for many metabolic processes, particularly in aerobic and anaerobic respiration. For example, oxidoreductases can be found in glycolysis, the TCA cycle and oxidative phosphorylation. Here we will focus on alcohol dehydrogenase which is an example frequently chosen by lecturers.
This enzyme interconverts alcohols to either aldehydes or ketones. In order for this to occur, NAD+ is reduced to NADH. This is similar to the majority of dehydrogenases that use NAD(P)+ or a flavin (such as FMN or FAD) as the electron acceptor. This enzyme is crucial in breaking down alcohol, removing the toxicity which is so important for many animals. In many bacteria, yeast and plants, alcohol dehydrogenase catalyses the reaction preferentially in the opposite direction. This is an important part of the fermentation process, as it used to maintain a constant supply of NAD+ in these organisms which used up during glycolysis.
What are transferases?
Transferases are enzymes that catalyse the movement of a functional group from one molecule to another. These functional groups are very diverse can include phosphate, methyl and glycosyl groups.
The basic reaction can be shown as:
AX + B --> A + BX
Where A= the donor, B= the acceptor and X= the functional group.
There are many transferase enzymes, here two sub-groups will be focused on.
Kinases enzymes are involved in catalysing the transfer of phosphate groups in a process called phosphorylation. They can act on a range of different molecules, for example lipids, carbohydrates and nucleotides. This is often occurs to prime the molecule ready for different metabolic pathways. Protein kinases are extremely important, as they are used extensively in signal transduction and in controlling complex processes within the cell. They are very diverse, with more than 500 different kinases being identified in the human body alone!
Another group of transferases are the deaminases, which catalyse the transfer an amine group. One of their roles is in the breakdown of amino acids after excess protein consumption. This reaction involves removing the amino group from the amino acid and then converting this to ammonia. The rest of the amino acid is then either oxidised for energy or recycled.
What are hydrolases?
Hydrolase enzymes simply catalyse hydrolysis; the breaking of single bonds through the addition of water.
There are a huge variety of hydrolase enzymes. For example, the digestive enzymes that are classified based on their target:
*proteases/ peptidases cleave peptide bonds between amino acids in order to breakdown proteins
*lipases break down lipids into fatty acids and glycerol by cleaving ester bonds
*nucleases cleave phosphodiester bonds between nucleotide subunits in nucleic acids
They are termed exo or endo depending on where they cut. Endo enzymes cut in the middle of the chain, whereas exo enzymes cut at the end of the chain to release an individual monomer.
What are lyases?
Lyases catalyse lysis reactions that generate a double bond. These are a type of elimination reaction but are not hydrolytic or oxidative. The reverse reaction catalyses an addition reaction, where a substrate is added to a double bond. These are often referred to as synthase enzymes.
An example lyase reaction would be:
ATP <--> cAMP + PPi
Generally one substrate is required in the forward direction, whereas two are needed for the backward reaction.
Catalyses the chemical reaction:
Oxalate + H+ <--> formate + carbon dioxide
This is an example of a decarboxylase or a carboxy-lyase, an enzyme that cleaves carbon-carbon bonds. It is involved glycoxylate and dicarboxylate metabolism.
This enzyme is also involved in the glycoxylate cycle, where it converts isocitrate to succinate. This is done through cleaving the glycoxylate group from isocitrate. It is technically an aldolase, as an aldol group in the form of glycoxylate is cleaved.
What are isomerases?
Isomerases are enzymes that can catalyse structural changes within a molecule. There is only one substrate and one product with nothing gained or lost, so they represent only a change in shape. The diagram shows a simple example of this sort of reaction.
Isomers have the same molecular formula but differ in their structural formula. These differences can change the chemical properties of the molecule. There are multiple classes of isomerases, for example geometric, structural, enantiomers and stereoisomers.
This enzyme converts the amino acid alanine between its two optical isomers. It is used in both alanine and aspartate metabolism.
D-Alanine <--> L-Alanine
All amino acids can exist in these two forms, except from glycine. The optical isomers are mirror images of each other, yet their prevalence in nature varies dramatically. The L-isomer is far more common, though the D-isomer amino acids do have some very important roles in biology (i.e. in peptidoglycan structure).
This enzyme catalyses the conversion of glucose-6-phosphate to fructose-6-phosphate in the second step of glycolysis.
Both glucose and fructose are 6-carbon sugars, but with a different structural arrangement. So this enzyme interconverts the sugar between its two forms.
What are ligases?
Ligases are responsible for the catalysis of ligation; the joining of two substrates. Usually chemical potential energy is required, so the reaction is coupled to the hydrolysis of a disphosphate bond in a nucleotide triphosphate such as ATP.
A very important ligase enzyme which will be focused on here is DNA ligase. It catalyses the ligation between breaks in DNA by forming a phosphodiester bond. There are different forms of the enzyme, and they catalyse different breaks. (In mammals there are 4 different types.) For example, double strand breaks are repaired by DNA ligase IV. Whereas DNA ligase I repairs single stranded breaks using the complementary strand as a template, like in DNA replication of the lagging strand. The reaction requires ATP, yet in some bacterial species the co-factor NAD has been shown to be a requirement.
1. Moss, G. (2011) Enzyme Nomenclature. www.chem.qmul.ac.uk/iubmb/enzyme Queen Mary University of London
2. Horton, R. Moran, L.A. Scrimgeour, G. Perry, M. Rawn, D. (2005) Principles of Biochemistry 4th Edition, Pearson Education (US)
3. Becker, W. Kleinsmith, L.J. Hardin, J. Bertoni, G.P. (2008) The World of the Cell, Benjamin Cummings
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