Introduction to Enzymes
Part II
Part II. Enzyme Types
Enzymes are biological catalysts. Like all catalysts, enzymes lower the energy needed to get a reaction started. Enzymes are much generally better at accelerating the rates of reactions than non-biological catalysts.
Figure 3. Diagram showing that less energy is required to get an enzyme catalyzed reaction started as compared to a non-catalyzed reaction.
Figure from Zubay et al., Principles of Biochemsitry copyright 1995 Brown Comm.
Enzymes have been divided into 6 classes by the International Commission on Enzyme Nomenclature. All enzymes are assigned a number (called an EC number) which defines exactly the reaction catalyzed by the enzyme. For example, trypsin is EC 3.4.21.4 since it is in class 3 (hydrolases) which work on peptide bonds (3.4) in the middle of proteins (3.4.21 are serine endopeptidases) - trypsin is the 4th entry in this subclass. Enzyme EC numbers can be looked up on the Web at http://expasy.hcuge.ch/sprot/enzyme.ht
These six classes are:
1. Oxidoreductases - enzymes catalyzing oxidation reduction reactions.
2. Transferases - enzymes catalyzing transfer of functional groups.
3. Hydrolases - enzymes catalyzing hydrolysis reactions.
4. Lyases - enzymes catalyzing group elimination reactions to form double bonds.
5. Isomerases - enzymes catalyzing isomerizations (bond rearrangements).
6. Ligases - enzymes catalyzing bond formation reactions couples with ATP hydrolysis.
These 6 enzyme classes can also be illustrated by the general reactions catalyzed:
Figure 4. Model reactions of the 6 classes of enzymes in the standard
Figure from Zubay et al., Principles of Biochemsitry copyright ©1995 Brown Comm.
Examples of enzymes in each class:
1. Alcohol dehydrogenase (EC 1.1.1.1)
2. Hexokinase (EC 2.7.1.1)
3. Trypsin (EC 3.4.21.4)
4. Ribulose-bisphosphate carboxylase (EC 4.1.1.39)
5. Triose phosphate isomerase (EC 5.3.1.1)
6. Tyrosine tRNA ligase (6.1.1.1)
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©Wilbur H. Campbell, 1995; wcampbel@mtu.edu