Enzymes
Biological catalysts that accelerate biochemical reactions by lowering activation energy, without being consumed in the process.
Enzymes are proteins (and in a few cases RNA molecules, called ribozymes) that catalyze biochemical reactions: they accelerate reactions by factors of millions to billions without being consumed. Every metabolic pathway — including cellular respiration, digestion, and DNA replication — runs on enzymes, which is why they are often called the workhorses of the cell. Their power comes from binding: an enzyme holds its substrate at a specific active site, orienting the molecules and stabilizing the transition state so that the activation energy drops sharply.
Specificity is the defining property. Each enzyme recognizes particular substrates and catalyzes particular reactions, because the active site has a shape and chemistry complementary to its substrate — the lock-and-key model, refined into the induced-fit model in which both enzyme and substrate adjust on binding. This is why enzymes are named for their substrates (lactase splits lactose, proteases split proteins) and why a single cell needs thousands of different enzymes.
Kinetics are described by the Michaelis–Menten equation, v = Vmax·[S]/(Km + [S]), where Km reflects how tightly the enzyme binds its substrate and Vmax reflects the maximum rate at saturating substrate. Enzymes are sensitive to conditions: temperature increases rates until heat denatures the protein, and each enzyme has an optimum pH — pepsin works in the stomach at pH ~2, while trypsin works in the intestine near pH 8 — tying enzyme function to the acid–base chemistry of its environment.
Regulation makes enzymes the control points of metabolism. Inhibitors block catalysis: competitive inhibitors compete for the active site, non-competitive ones bind elsewhere and distort the enzyme. Cells use feedback inhibition — the end product of a pathway inhibits its first enzyme — and allosteric regulation to switch pathways on and off. Many drugs and poisons are enzyme inhibitors: aspirin inhibits cyclooxygenase, and many antibiotics inhibit bacterial enzymes that have no human counterpart.
Tags
biochemistry catalysis metabolism proteins
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