78 Steps Health Journal

Substrate Concentration

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Substrate concentration may be altered as a result of factors that alter the supply of a substrate from outside a cell. For example, there may be changes in its blood concentration due to changes in diet or rate of substrate absorption from the intestinal tract. In addition, a substrate's entry into the cell through the plasma membrane can be controlled by mechanisms that will be discussed in Chapter 6. Intracellular substrate concentration can also be altered by cellular reactions that either utilize the substrate, and thus lower its concentration, or synthesize the substrate, and thereby increase its concentration.

The rate of an enzyme-mediated reaction increases as the substrate concentration increases, as illustrated in Figure 4-10, until it reaches a maximal rate, which remains constant despite further increases in substrate concentration. The maximal rate is reached when the enzyme becomes saturated with substrate—that is, when the active binding site of every enzyme molecule is occupied by a substrate molecule.

Since coenzymes function as substrates in certain enzyme reactions, changes in coenzyme concentration also affect reaction rates, as occurs with ordinary substrates.