Enzymes: How Chemical Digestion Works

A cracker that turns sweet

Chew a plain unsalted cracker without swallowing for about a minute. Many people notice it starts to taste slightly sweet. Nothing sweet was added. Your saliva contains an enzyme, salivary amylase, that is already hydrolyzing the cracker's starch into maltose, a sugar.

Without enzymes, the hydrolysis of starch at body temperature would be far too slow to keep you alive. Enzymes make digestion (and almost every other reaction in your cells) fast enough to matter.

How enzymes work

Every reaction needs a push to get started, called its activation energy. An enzyme is a catalyst: it lowers the activation energy, so the reaction happens much faster at body temperature. The enzyme is not used up; one enzyme molecule can catalyze the same reaction thousands of times.

1. The substrate binds to the enzyme's active site, a pocket with a particular shape and chemistry.
2. In the induced-fit model, the active site changes shape slightly to grip the substrate tightly, forming an enzyme-substrate complex. This strains the substrate's bonds, making them easier to break (or form).
3. The products are released, and the enzyme is ready again.

Because the active site fits only certain molecules, each enzyme is specific: amylase digests starch but not protein. (An older, simpler model, lock and key, imagines a rigid site that fits the substrate exactly; induced fit is the more accurate model.) Most enzyme names end in -ase, often after the substrate: lipase digests lipids, lactase digests lactose.

What affects enzyme activity?

• Temperature. Warming speeds molecules up, so they collide more often, and the rate rises. Above the optimum (about 37 °C for most human enzymes), the enzyme's shape starts to break down: it denatures, and the rate falls quickly. Cooling only slows an enzyme; it usually works again when warmed. Denaturing by heat is usually permanent.
• pH. Each enzyme has an optimum pH. Pepsin works in the very acidic stomach (optimum about pH 2). Trypsin works in the small intestine, where the pH is slightly basic (optimum about pH 8). Salivary amylase works best near neutral (about pH 7). Far from the optimum, the active site changes shape.
• Substrate concentration. More substrate means more collisions, so the rate rises, until every active site is busy. Then the enzyme is saturated and the rate levels off.
• Enzyme concentration. More enzyme gives a faster rate, as long as there is plenty of substrate.
• Inhibitors. Some molecules block or change the active site. Many medicines work this way.

The digestive enzymes

Digestive enzymes work outside cells, in the space inside the gut. Each class of macromolecule has its own enzymes:

• Carbohydrates: salivary amylase (mouth) and pancreatic amylase (small intestine) hydrolyze starch to maltose. Enzymes on the surface of the intestinal lining (the brush border) finish the job: maltase (maltose → glucose + glucose), sucrase (sucrose → glucose + fructose) and lactase (lactose → glucose + galactose).
• Proteins: pepsin (stomach) cuts proteins into shorter polypeptides. Trypsin and chymotrypsin (from the pancreas) cut them into shorter peptides. Peptidases on the brush border finish them into amino acids.
• Lipids: pancreatic lipase hydrolyzes triglycerides into fatty acids and monoglycerides (glycerol with one fatty acid still attached). Bile helps, as you'll see in the next lesson.
• Nucleic acids: nucleases from the pancreas break DNA and RNA into nucleotides.

Why don't protein-digesting enzymes digest the cells that make them? They are made as inactive forms. Pepsinogen becomes pepsin only when it meets stomach acid. Trypsinogen becomes trypsin only in the small intestine, activated by an enzyme on the intestinal lining (enterokinase).