The workers' assembly rate of airplane tails is satisfactory; the supervisor is content. The tails are not being used as fast as they're made and they begin to accumulate; the supervisor takes note. The supervisor calls for a slowdown. The inventory of finished tails decreases as workers slow production. Later, as the supply of tails falls, the supervisor signals the workers to speed up their rate of assembly.

Lines and Loops An assembly line moves in one direction only - from input of raw materials to output of product, with the supervisor acting as the governor, or controlling agent. If too much product begins to accumulate, the supervisor slows down the input of raw materials. Conversely, if there are too many raw materials, the supervisor speeds up production.

To appreciate how feedback works, it helps to imagine the information (the signals that say "too much" or "not enough") as flowing in a loop. Bending the production line into a circle and stationing the supervisor at a strategic point point overseeing both input and output gives him or her greater control. This arrangement is impractical for many factories, but it works beautifully inside cells, in molecular assembly lines like the one shown below.

In a negative feedback loop, information flows in a circle, which is central to controlling the rate of assembly. A single supervisor is the key to the information loop's continuity. Here are four enzymes making an amino acid.

The first enzyme - the supervisor - notes an excess build-up of the amino acid. He stops the assembly line and waits for inventory to drop. When the amino acid level is low enough, he starts the line again.

Enzyme Governors We can now better appreciate why we call enzymes "smart."Their unique chemical character gives them the ability to do their usual work of rearranging or disassembling other molecules but also to process information. Certain supervisory, or regulatory, enzymes do this by readily and reversibly changing shape in response to a signal. In addition to the working site on their surface where other molecules "dock" to get processed, regulatory enzymes have a second site specifically designed to hold a small signal molecule. Nestled in this special niche, the signal molecule acts like a finger on an on/off switch: It causes the enzyme to modify its shape so that its working site stops functioning. Allostery (literally "other shape"), the name given to this almost ridiculously simple behavior, underlies most of the unimaginably complex regulatory processes of life. Imagine a tightly balled string of beads surrounding a marble. Now imagine pushing your finger into the beads on the side opposite the marble, causing the beads to shift. As your finger moves into its niche, the marble pops out. Regulatory proteins behave in a similar way. When a signal molecule enters one site it it changes the function of another site.

All material copyright © 1995 Times Books/Random House