Unit 7
Enzymes

Overview

The thousands of biochemical reactions that go on in vivo are not random events, but rather are controlled by biological catalysts called enzymes. Besides their effect on biological order and control, enzymes have evolved so that the reactions they catalyze can occur quickly under the mild conditions that exist in vivo.

Enzymes, as you learned in Unit 2, are one of the subclasses of proteins. Enzymes are made up of stereospecific amino acids (remember that only L-amino acids are found in proteins); therefore, enzymes themselves are stereospecific. This characteristic means that the enzyme glucose oxidase, for example, will oxidize D-glucose but not L-glucose. In addition, most enzymes have a small, non-protein group attached either covalently or noncovalently. These attached groups, which are the actual sites of catalysis, are called “coenzymes.”

In Unit 7, we discuss the structure and some functions of enzymes and co-enzymes. The unit is divided into seven lessons:

  1. Enzymes and Coenzymes
  2. Enzyme Classification
  3. The Chemistry of Enzymatic Reaction Mechanisms
  4. Enzyme Kinetics I—Introduction
  5. Enzyme Kinetics II—The Michaelis-Menten Equation
  6. Enzyme Inhibition
  7. Allosteric Enzymes

Objectives

After completing this unit, you should be able to

  1. define “enzyme” and “coenzyme,” and explain the relationship between the two.
  2. describe the six classes of enzymes.
  3. explain the specificity of enzymes.
  4. list the types of chemical reactions involved in enzyme catalysis.
  5. define “enzyme kinetics,” “enzyme inhibition,” and “Michaelis-Menten enzyme.”
  6. use graphical techniques to analyse and describe simple, unimolecular enzyme activities.
  7. explain the difference between Michaelis-Menten and allosteric enzymes.
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Glossary

allosteric enzyme a regulatory enzyme whose affinity for its substrate is affected by the presence or absence of other molecules
apoenzyme protein portion of an enzyme (i.e., lacking a coenzyme)
enzyme protein which catalyzes a biochemical reaction in vivo at 37°C, 0.1 M salt, and room pressure; frequently an enzyme name ends in -ase (e.g., amylase and carbonic anhydrase)
holoenzyme complete active enzyme (i.e., protein + coenzyme)
enzyme classification assignment of an enzyme to one of six groups, depending on the type of chemical reaction which the enzyme catalyzes
first order kinetics rate of reaction is directly proportional to the concentration of starting materials (i.e., substrate)
inhibition alteration in an enzyme’s activity, usually caused by modification of the enzyme active site, so that substrate cannot bind to the enzyme, or substrate can bind but cannot be converted to product, or product cannot be released
Michaelis-Menten kinetics simple mathematical description of a first-order enzyme reaction [Leonor Michaelis was British and Maud Menten was Canadian]
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