All Physiological Change Is Mediated by Proteins

All physiological change is mediated by a single class of polymeric macromolecules (large molecules), the proteins. Protein function can be subdivided into a number of categories: catalysis, reaction coupling, transport, structure, and signaling.

Catalysis is the ability to increase greatly the rate of a chemical reaction without altering the equilibrium of the reaction. The vast majority of biochemical reactions occur at a physiologically useful rate only because of protein catalysts, called enzymes. Examples of enzymatic catalysis in the synthesis of a class of physiological regulator molecules, catecholamines, are given later in this chapter.

In reaction coupling, two reactions are joined together with the transfer of energy. Energy from a spontaneous reaction (similar to water flowing downhill) is tunneled to a non- spontaneous reaction (e.g., sawing wood) so that the sum of the two reactions is spontaneous. That is, the energy liberated by the “downhill* reaction is used to drive the “uphill" reaction. This is the basic function of a motor; the “downhill” burning of gasoline is coupled with the “uphill” movement of the car. The ability of proteins to couple spontaneous and nonspontaneous reactions allows cells to be chemical motors, using chemical energy to do various jobs of work. One such job of work, the contraction of striated muscle, is discussed later with particular emphasis on the proteins involved.

Proteins provide a pathway for the transport of most molecules and all ions into and out of the cell. Transport and transport proteins are discussed more fully after a discussion of the lipid bilayer membrane, the major obstacle to transport.

Proteins that form filaments and that glue cells to each other and to their environment are responsible for the structure and organization of cells and of multicellular assemblies (i.e., the tissues and organs of animals).

At its most basic level, signaling requires only a controlled change or difference. Human signaling occurs by way of open and closed electrical circuits (telegraphy), puffs of smoke in the air, and complex black marks on a contrasting background (numbers and letters). As discussed next, a fundamental property of proteins is the ability to change shape. The cell can use changes of protein shape directly to send signals, and the function of some proteins is purely informational. That is, all that some proteins do by changing shape is transmit and transduce information. Information can be defined as "any difference that makes a differenceor more simply, "any dif­ference that regulates something.” Catalysis, coupling, trans­port, structural, and signaling functions can be combined on individual protein molecules. As will become apparent, such multifunctional proteins carry out many important physio­logical functions. Also important is that a change in one or more of these protein functions can be used to carry informa­tion, to serve as a signal within the cell. Thus, in addition to proteins specialized exclusively to carry information, changes in enzymatic activity or ion transport can also “make a difference,” transmitting information and triggering an appropriate response.