Enzymes

Your body runs on chemistry. Digesting a meal, building muscle, pulling energy out of sugar: all of it is chemical reactions. Left to themselves, most of those reactions would happen far too slowly to keep you alive.

Enzymes fix that. An enzyme is a protein folded into a particular shape, with a pocket that fits one specific molecule, a bit like a key fitting a lock. It grabs that molecule, holds it in just the right position to make the reaction happen, then lets the product go and starts again.

One enzyme can do this thousands of times a second. It is not used up in the process, so the same enzyme keeps working over and over. And each type only does its own job. The enzyme that breaks down starch will not touch fat.

Almost all enzymes are proteins, and what they do is lower the activation energy of a reaction. Every reaction has an energy barrier the molecules must climb before they can react. Enzymes make that barrier smaller, so far more molecules get over it each second. Note the limit here: an enzyme does not change whether a reaction wants to happen. It only changes how fast the reaction reaches the same balance point it would have reached anyway, and it comes out the other side unchanged, ready to go again.

The molecule an enzyme works on is its substrate, and it binds in a region called the active site. The old picture was lock-and-key: the substrate slots into a rigid, perfectly matched pocket. The better picture is induced fit. The active site is a little loose, and it moulds itself around the substrate as it binds, tightening its grip and straining the substrate toward its reacted shape.

How fast an enzyme works depends on a few things. More substrate means a faster rate, until every enzyme is busy and adding more makes no difference. Warmth speeds things up too, up to a point. Push the temperature too high, or the acidity too far from what the enzyme likes, and the protein loses its careful shape. This is called denaturing: the enzyme unfolds, the active site falls apart, and it stops working, often for good.

Many enzymes need help. Small non-protein partners called cofactors, and larger organic ones called coenzymes, sit in the active site and do chemistry the protein alone cannot manage. A lot of the vitamins in your diet exist to become coenzymes.

Enzymes can also be switched off by inhibitors. A competitive inhibitor looks enough like the substrate to sit in the active site and block it. A non-competitive inhibitor binds somewhere else and bends the enzyme out of shape. This is not just biology trivia. A huge share of medicines, and many poisons, work by inhibiting one specific enzyme.

Transition-state stabilisation

The reason an enzyme lowers the activation barrier was pinned down by Linus Pauling. The active site is shaped to bind the transition state, the fleeting strained arrangement at the very top of the barrier, more tightly than it binds the substrate itself. By stabilising that high-energy state, the enzyme pulls the barrier down. This is also why the best enzyme inhibitors are often transition-state analogues, molecules built to mimic that shape and jam the site.

Michaelis-Menten kinetics

Treat the enzyme \(E\) and substrate \(S\) as forming a complex \(ES\) that either falls back apart or turns into product. Under the steady-state assumption the rate follows \[ v = \frac{V_{max}\,[S]}{K_m + [S]}. \] Here \(V_{max} = k_{cat}[E]_T\) is the ceiling reached when every enzyme is saturated, \(k_{cat}\) is the turnover number, the products made per enzyme per second, and \(K_m\) is the substrate concentration at half of \(V_{max}\). A low \(K_m\) means the enzyme still grabs substrate even when it is scarce.

Catalytic efficiency

The ratio \(k_{cat}/K_m\) measures how good an enzyme is at the low substrate levels where it spends most of its working life. It is capped by how fast enzyme and substrate can even bump into each other by diffusion, around \(10^{8}\) to \(10^{9}\) per molar per second. Enzymes that reach this ceiling, such as catalase and triosephosphate isomerase, are called catalytically perfect. They react with almost every substrate molecule they meet.

How the chemistry gets done

Active sites use a handful of tricks, usually several at once. In acid-base catalysis, side chains donate or accept protons at just the right moment. In covalent catalysis, the enzyme forms a temporary covalent bond with the substrate to route it through an easier path. In metal-ion catalysis, a bound metal such as zinc or magnesium stabilises charge or activates a water molecule. Lining the reacting groups up precisely, and squeezing out water, does much of the rest.

Regulation and cooperativity

Cells tune their enzymes on the fly. Allosteric enzymes carry a second site, away from the active site, where a regulator binds and reshapes the protein. When a multi-subunit enzyme binds substrate cooperatively, its rate curve turns sigmoidal rather than the plain hyperbola of Michaelis-Menten, giving a sharp switch-like response. Feedback inhibition uses the end product of a pathway as an allosteric brake on the first enzyme, so the cell stops making what it already has enough of. Competitive inhibitors raise the apparent \(K_m\) while leaving \(V_{max}\) untouched, since a flood of substrate outcompetes them, whereas classic non-competitive inhibitors lower \(V_{max}\) without changing \(K_m\).

Not all enzymes are proteins

Some enzymes are made of RNA. These ribozymes, including the catalytic core of the ribosome that stitches every protein together, show that RNA can both carry information and do chemistry. That dual talent is a central clue in the origin of life, hinting at an early RNA world where the same molecules stored genes and ran reactions, long before proteins took over most of the catalysis.