What Proteins Do

Life's diversity can be traced to differences in the kinds and arrangements of protein molecules. More than half of the non-water weight of your cells is protein. Proteins do the daily business of living, giving cells their shapes and unique abilities. We've alluded to some of proteins' abilities earlier. Here's more about the key roles they play.

	Enzymes Enzymes are catalysts - they speed up the breaking apart and putting together of molecules. Their surfaces have special shapes that "recognize" specific molecules, similar to the way a lock accepts only a certain key. Enzymes themselves remain unchanged by the changes they bring about; so they can be used over and over again.
Transporters Special transporter proteins in cell membranes function as tunnels and pumps, allowing material to pass in and out of cells.
	Movers Because the shape of protein chains is mostly determined by weak, easily broken and remade chemical bonds, these chains can shorten, lengthen, and change shape in response to the input or withdrawal of energy. The energy molecule ATP can activate one part of a protein molecule, causing another part of the same molecule to slide or take a "step." Subsequent removal of ATP causes the protein to return to its original shape, taking another step in the process. Then the cycle can be repeated.
Supporters Long chains of folded or coiled proteins can form sheets and tubes - the cell's equivalent of posts, beams, plywood, cement, and nails.
	Regulators Enzymes that convert one chemical to another must do so in several steps. The first enzyme in a cycle "notices" when enough of the final product builds up and shuts down the assembly line. This ability to respond to feedback is built into the regulator's structure (see question 6).
Communicators To work together in harmony, cells must be able to pass messages back and forth. Proteins act as cells' chemical messengers. Hormones are examples. Communicator proteins sit on the surface of the receiving cell to gather the incoming signal.
	Defenders Antibodies are proteins with special shapes that recognize and bind to foreign substances, such as bacteria or viruses, surrounding them so that scavenger cells can destroy them and flush them out of the body.