Enzymes

Enzymes are nature's catalysts. You have already met them as proteins that speed up the chemistry in your body. Your cells hold thousands of different enzymes, and each one speeds up one reaction, or a few closely related ones.

Catalase is one. Your cells make a little hydrogen peroxide as they work, and too much of it damages them. Catalase breaks it down into water and oxygen, the reaction from the last section, at an astonishing speed: one catalase molecule can break down millions of hydrogen peroxide molecules every second. Liver is rich in catalase, and a small piece of raw liver dropped into hydrogen peroxide froths with oxygen.

The active site

An enzyme is a long chain of amino acids folded up, like every protein. The folding leaves a small pocket in its surface called the active site. The groups that line the pocket fit one kind of molecule and hold it in just the right position, so the bond that has to break sits right next to the groups that help break it. That gives the reaction its lower hill. When the reaction is done, the products leave, and the active site is free for the next molecule.

Earlier you chewed bread until it tasted sweet. That was amylase in your saliva at work. Its active site holds a stretch of starch chain and cuts one link, using a water molecule: it runs the condensation that built starch in reverse. Cut after cut, the starch breaks down into maltose, malt sugar, which tastes sweet.

Too hot for an enzyme

The enzymes in your body work best at about 37 °C, your body temperature. Heat one much more and it stops working. Hydrogen bonds, like those in water, help hold the protein chain folded the right way. Heat shakes them apart, the chain unfolds, and the active site loses its fit. The egg white that turns firm in a hot pan is protein unfolded in just this way.

Lesson 19 looks at how medicines fit pockets like these in the body's own proteins.