Enzymes and Their Classification

Enzymes (1)

• Enzymes are protein catalysts that speed up chemical reactions without being consumed.

Enzyme Classification

• Most enzymes are named by adding the suffix "-ase" to the name of their substrate or a descriptive term for the reaction they catalyze.
• Urease has urea as a substrate.
• Alcohol dehydrogenase catalyzes the removal of hydrogen from alcohols.
• International Union of Biochemistry and Molecular Biology (IUBMB) categorizes enzymes according to the general class of organic chemical reaction they catalyze.
• The IUBMB scheme assigns a unique EC number to each enzyme.

Six Classes of Enzymes

• Oxidoreductases: Catalyze oxidation-reduction reactions. Commonly called dehydrogenases, oxidases, peroxidases, or oxygenases, reductases.
  • Lactate dehydrogenase is an example (EC 1.1.1.27) converting L-Lactate to pyruvate.
• Transferases: Catalyze group transfer reactions. Some require coenzymes.
  • Kinases transfer a phosphoryl group from ATP.
  • Alanine transaminase (EC 2.6.1.2) transfers an amino group from L-alanine to alpha-ketoglutarate.
• Hydrolases: Catalyze hydrolysis reactions.
  • Pyrophosphatase (EC 3.6.1.1) is an example.
• Lyases: Catalyze the cleavage of C-C, C-S, and C-N bonds, resulting in the formation of double bonds.
  • Pyruvate decarboxylase (EC 4.1.1.1) splits pyruvate into acetaldehyde and carbon dioxide.
• Isomerases: Catalyze structural changes within a single molecule (isomerization reactions).
  • Alanine racemase (EC 5.1.1.1) converts L-alanine to D-alanine.
• Ligases: Catalyze the joining of two substrates, requiring the input of chemical potential energy (e.g., from ATP). Often referred to as synthetases.
  • Glutamine synthetase (EC 6.3.1.2) joins glutamate and ammonia to produce glutamine.

Holoenzymes

• Some enzymes require molecules (other than proteins) for activity, called coenzymes or cofactors.
• The active enzyme with its nonprotein component is a holoenzyme.
• Apoenzyme refers to the enzyme without its non-protein component, which is inactive.
• Metal ion cofactors (e.g., Zn2+, Fe2+)
• Small organic molecules (coenzymes).

Vitamins as Coenzymes

• Several B vitamins act as coenzymes in enzymatic reactions.

Active Sites

• Active sites are complex and not passive. They employ diverse chemical mechanisms to facilitate substrate conversion to product.

Catalytic Efficiency

• Enzyme-catalyzed reactions proceed much faster than uncatalyzed reactions (10³ to 10⁸ times faster).
• Turnover number (k) represents the number of substrate molecules converted to product per enzyme molecule per second. Typically 10² to 10⁴ s⁻¹.

Enzyme Specificity

• Enzymes are highly specific, interacting with one or a few substrates and catalyzing only one type of reaction.

Enzyme Regulation

• Enzyme activity can be regulated (increased or decreased) to meet cellular needs.

Proenzymes/Zymogens

• Some enzymes are produced in an inactive form, called proenzymes or zymogens.
• Zymogens are converted into active forms through the cleavage of peptide bonds.
• Chimotrypsinogen is an example.

Multienzyme Systems

• Several enzymes can work together in a pathway.
• Substrates are passed along to different enzymes in the pathway.

Energy Changes in Reactions

• Chemical reactions have an energy barrier (activation energy) that enzymes lower.
• This allows reactions to occur faster.

Factors Affecting Enzyme Reaction Rates

• Substrate Concentration: Increasing substrate concentration increases reaction rate until saturation is reached.
• Temperature: Reaction velocity increases with temperature up to a point. Beyond that, the enzymes can denature and lose activity. Optimal temperature varies by enzyme.
• pH: The pH affects the ionization of active site groups, which affects enzyme function. Optimal pH varies by enzyme.

This quiz covers the basics of enzymes, including their role as protein catalysts, naming conventions, and classification according to the IUBMB. Explore the six classes of enzymes and their specific functions in biochemical reactions.