What an enzyme does

Enzymes are the cell's catalysts — almost always proteins. One can speed a reaction up by a million-fold or more, come out unchanged at the end, and it is exquisitely specific for its substrate.

The active site & induced fit

The substrate binds a small, precisely-shaped pocket — the active site. It's not a rigid lock: as the substrate settles in, the enzyme closes around it (induced fit), squeezing it toward the transition state. That snug grip is where the specificity — and the catalysis — comes from.

Six classes of enzymes

The Enzyme Commission sorts every enzyme into six classes — by the reaction it catalyzes, not by what it acts on.

Naming them

Most enzyme names tell you the class: they end in –ase and often name the substrate or reaction — lactase splits lactose, DNA polymerase builds DNA, alcohol dehydrogenase removes hydrogens.

How fast does an enzyme go?

Measure the initial rate (V₀) at different substrate levels. At low [S] the rate climbs steeply — plenty of free enzyme. At high [S] it flattens out: every enzyme is busy, so the enzyme is saturated and can't go faster.

What Vmax and Kₘ tell you

• Vmax — flat-out speed; more enzyme (or a faster k꜀ₐₜ) raises it.
• Kₘ — the [S] at half-speed. Low Kₘ = high affinity (the enzyme grabs substrate even when it's scarce).

Straightening the curve

The Michaelis–Menten curve is a hyperbola — and Vmax is that ceiling you only approach, so it's hard to pin down by eye. The fix: take the reciprocal of both sides. A hyperbola flips into a straight line, and a line is easy to read exactly.

Slowing an enzyme down

An inhibitor is any molecule that makes an enzyme run slower. The two classic reversible types differ only in where they bind:

• Competitive — binds the active site, fighting the substrate for the same spot. Pile on more substrate and you out-compete it.
• Noncompetitive — binds somewhere else and bends the enzyme out of shape. More substrate can't rescue it — the spare enzyme is just broken.

Regulators that never touch the active site

Allosteric enzymes have a second pocket — the allosteric site. When a regulator binds there, the whole enzyme changes shape, and every active site feels it. These enzymes usually have multiple subunits that bind substrate cooperatively, just like hemoglobin binds O₂.

A reversible switch: phosphorylation

Attach a phosphate to a Ser, Thr, or Tyr and the enzyme's activity flips. A kinase adds it (spending ATP); a phosphatase takes it back off — so the switch runs both ways, on demand.

A one-way switch: zymogens

Some enzymes are made as inactive precursors — zymogens. A single proteolytic cut snips out a piece and locks them on for good. There's no going back — you can't glue the peptide back on — so it's perfect for a commit-once signal.

Meet chymotrypsin: the gut's scissors

Chymotrypsin is a serine protease — it snips peptide bonds using one supercharged serine. And it's picky: it prefers to cut on the carbonyl side of the big aromatics — Phe, Tyr, Trp — whose fat rings drop into a deep pocket.

Half 1: acylation

Ser–O⁻ slams into the carbonyl carbon → a shaky tetrahedral intermediate whose negative oxygen is hugged by the oxyanion hole. The peptide breaks, the first product (the new amino end) walks off, and the enzyme is left wearing the acyl piece.

Half 2: deacylation

Now water finishes the job. His57 pulls a proton off it so it can attack the acyl-enzyme — a second tetrahedral intermediate in that same oxyanion hole — and the second product leaves as a –COOH. Ser–OH is reborn, ready to cut again.