Enzyme Definition and Mechanism of Action

Study Notes

Enzymes are primarily protein catalysts that accelerate reaction rates.
They are synthesized intracellularly but can function extracellularly.
Enzymes remain unchanged during reactions and direct cellular metabolic processes.
The lock-and-key model describes enzymes with rigid active sites, fitting only specific substrates.
The induced-fit model proposes flexible active sites that adjust shape to bind substrates, allowing broader substrate specificity.

All enzymes are proteins, except ribozymes (RNA).
Each enzyme has an active site where substrates bind, forming an enzyme-substrate complex (ES) that converts to an enzyme-product complex (EP), ultimately releasing the product.
Enzymes exhibit high catalytic efficiency, converting hundreds to thousands of substrate molecules per second.
Enzymes are highly specific, interacting with one or a few substrate types and catalyzing specific reactions. Examples include urease acting on urea and amylase on glycogen/starch.
Holoenzymes consist of an apoenzyme (protein) and a non-protein cofactor (metal ion or coenzyme). Apoenzymes are inactive without the cofactor. Coenzymes can be derived from vitamins (e.g., NAD+ contains niacin).
Enzyme activity is regulated through activation or inhibition.
Enzymes are compartmentalized within cells for efficient and isolated reactions.

Each enzyme has an optimal pH.
Enzyme activity is influenced by blood pH levels.
Substrate concentration directly impacts reaction rate.
The Michaelis constant (Km) represents the substrate concentration at half-maximal velocity (Vmax). A low Km indicates high substrate affinity; a high Km indicates low substrate affinity.
Enzyme concentration increases reaction rate (at constant substrate concentration).
Inhibitors decrease enzyme activity; reversible inhibitors bind non-covalently, irreversible inhibitors bind covalently.

Competitive inhibitors structurally resemble the substrate, bind to the active site, compete with the substrate, increase Km, and are reversible by increasing substrate.
Noncompetitive inhibitors differ structurally from the substrate, bind to a site other than the active site, change enzyme shape and active site, are not reversible by adding substrate, and do not alter Km.

General regulation involves increasing enzyme activity by raising substrate concentration.
Special regulatory mechanisms include allosteric effectors (binding to sites other than the active site, altering enzyme shape and activity), covalent modification (like phosphorylation, which can activate or inactivate), altering enzyme synthesis rates (slow, gene-level), and feedback product inhibition.

Blood enzymes are categorized into those with functions in blood (e.g., coagulation factors) and those without (released during cell turnover).
Elevated levels of enzymes normally present in cells (but not functional in blood) indicate tissue damage.
Enzyme levels are used to diagnose diseases affecting the heart, liver, muscles, and other tissues.
Some enzymes are highly specific to particular organs; their elevated levels are diagnostic for diseases of that organ (e.g., alanine aminotransferase (ALT) for liver damage).