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Chapter 9: Enzymes and Enzyme Kinetics

Introduction

High-Yield Terms

  • Simple enzyme: binds only substrate and/or inhibitors

  • Holoenzyme: complex enzymes which are composed of protein plus a relatively small organic molecule

  • Apoenzyme: the protein component of a complex enzyme

  • Coenzymes: nonprotein components of enzymes

  • Isozyme: member of a set of enzymes sharing similar/same substrate specificity

  • Metalloenzyme: enzymes that require a metal in their composition

  • Ribozymes: RNA molecules that exhibit catalytic activity in the absence of any protein component

  • Reaction rate: described by the number of molecules of reactant(s) that are converted into product(s) in a specified time period

  • Chemical reaction order: relates to the number of molecules in a reaction complex that can proceed to product

  • Equilibrium constant: related to the ratio of product concentration to reactant concentration or rate constants for forward and reverse reactions

  • Michaelis-Menten constant (Km): relates the maximum reaction velocity to substrate concentration such that Km is the substrate concentration [S] at which a reaction has attained half the maximal velocity

  • Lineweaver-Burk plots: linear double reciprocal plots of substrate concentration versus reaction velocity

  • Allosteric enzymes: defined as any enzyme whose activity is altered by binding small molecule regulators referred to as effectors; plots of substrate concentration versus reaction velocity are sigmoidal instead of curvilinear

Enzymes are biological catalysts responsible for supporting almost all of the chemical reactions that maintain animal homeostasis. Because of their role in maintaining life processes, the assay and pharmacological regulation of enzymes have become key elements in clinical diagnosis and therapeutics. The macromolecular components of almost all enzymes are composed of protein, except for a class of RNA-modifying catalysts known as ribozymes. Ribozymes are molecules of ribonucleic acid that catalyze reactions on the phosphodiester bond of other RNAs (see Chapter 35) (Table 9-1).

TABLE 9-1:International Union of Biochemistry (IUB) Classification of Enzymes
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TABLE 9-1: International Union of Biochemistry (IUB) Classification of Enzymes
Number Classification Biochemical Properties
1 Oxidoreductases Act on many chemical groupings to add or remove hydrogen atoms
2 Transferases Transfer functional groups between donor and acceptor molecules. Kinases are specialized transferases that regulate metabolism by transferring phosphate from ATP to other molecules
3 Hydrolases Add water across a bond, hydrolyzing it
4 Lyases Add water, ammonia, or carbon dioxide across double bonds, or remove these elements to produce double bonds
5 Isomerases Carry out many kinds of isomerization: L to D isomerizations, mutase reactions (shifts of chemical groups), and others
6 Ligases Catalyze reactions in which 2 chemical groups are joined (or ligated) with the use of energy from ATP

Enzyme Classifications

Enzymes are also classified on the basis of their composition. Enzymes composed wholly of protein are known as simple enzymes in contrast to complex enzymes, which are composed of protein plus a relatively small organic molecule. Complex enzymes are also known as holoenzymes. In ...

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