Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
6.1 An Introduction to Enzymes
This chapter introduces enzymes as the essential biological catalysts of life, covering their discovery, protein nature, use of cofactors and coenzymes, classification, and their extraordinary power and specificity.
The Discovery and History of Enzymes
• The study of enzymes began with research on digestion and fermentation in the 18th–19th centuries. Eduard Buchner’s 1897 experiment with yeast extracts proved that catalysis could occur outside living cells, ending the belief that vital “life force” was required.
• Frederick W. Kühne coined the term “enzyme.” In 1926, James Sumner crystallized urease, confirming that enzymes are proteins; further work by Northrop and Kunitz solidified this understanding.
• By the late 20th century, thousands of enzymes had been purified, their structures solved, and mechanisms explained. Enzymes can increase reaction rates by factors as great as 1017 (e.g., orotidine phosphate decarboxylase).
The Nature and Composition of Enzymes
• Almost all enzymes are proteins (except a few catalytic RNAs). Their catalytic activity depends on their precise three-dimensional structure, and denaturation or dissociation usually destroys activity.
• Enzymes have a wide range of molecular weights (from ~12,000 to over 1,000,000 Da) and structural complexity (primary to quaternary structure).
• Some enzymes function with only their amino acid residues; others require additional nonprotein components called cofactors—either inorganic ions (e.g., Fe2+, Mg2+, Mn2+, Zn2+) or organic molecules called coenzymes.
• Coenzymes act as transient carriers of specific functional groups and are usually derived from vitamins (see Table 6-2). Some enzymes need both a coenzyme and metal ion for activity.
• A tightly or covalently bound coenzyme or metal is called a prosthetic group. The complete, catalytically active enzyme with its cofactors is a holoenzyme; the protein part alone is called an apoenzyme.
• Enzymes can also be covalently modified (phosphorylation, glycosylation, etc.), which often regulates activity.
Classification and Nomenclature of Enzymes
• Many enzymes are named with the suffix “-ase” added to their substrate or function (e.g., urease, DNA polymerase). Others have traditional names (e.g., pepsin, trypsin, lysozyme).
• To reduce ambiguity, enzymes are systematically classified into seven classes based on the type of reaction they catalyze. Each enzyme is assigned a unique four-part Enzyme Commission (E.C.) number and a systematic name describing its function.
• The seven classes are:
1. Oxidoreductases: transfer electrons (hydride ions or H atoms)
2. Transferases: transfer groups between molecules
3. Hydrolases: hydrolysis reactions (transfer of functional groups to water)
4. Lyases: cleavage of various bonds (C–C, C–O, C–N, etc.) by elimination or addition
5. Isomerases: transfer groups within molecules to yield isomeric forms
6. Ligases: formation of bonds (C–C, C–S, C–O, C–N) via condensation reactions coupled to ATP cleavage
7. Translocases: movement of molecules or ions across membranes or their separation within membranes
• A complete list of enzymes is maintained by the International Union of Biochemistry and Molecular Biology (IUBMB).
• Enzymes may have both a systematic and a commonly used trivial name.
In a Nutshell
• Enzymes are highly efficient, specific biological catalysts—almost all are proteins—that drive nearly every reaction in life.
• Many require cofactors or coenzymes for full activity and can be regulated by covalent modification.
• Enzymes are classified by the reactions they catalyze, with formal E.C. numbers and names aiding clear identification.