Multiple Substrates

In the case where two substrates, S1 and S2, bind reversibly to a enzyme to make a single product, we may assume the following reaction mechanism:

Si + ES2 ES1S2

k-21

where the first four reactions are in equilibrium with dissociation constants of K1, K2, K12, and K21, respectively. The resulting equation for the rate is r=¬ęKOTrrb (541)

where

It is interesting that an empirical equation of the same form as (5.41) is sometimes used to model dual substrate microbial kinetics, except that K* is treated as a constant. For example, such a model has been used where the two substrates are chemical oxygen demand (COD) and oxygen, or COD and nitrate.

Besides the substrate inhibition described above, inhibition can come from compounds other than the substrate. Some inhibitors will react irreversibly with the enzyme, removing it from availability permanently. These are poisons. An example is cyanide, which deactivates an enzyme in the respiration process. Others act by competing with substrate in reversible reactions with the enzyme. Their effect can be categorized in terms of their effect on the coefficients of the Michaelis-Menten equation and in terms of how they bind to the enzymes.

Competitive inhibitors bind to the same active site as the substrate. They do not change the maximum reaction rate, but do change Km. Their effect can be overcome by increasing substrate concentration. For example, TCE is cometabolized with substrates such as methanol. The presence of TCE reduces the biodegradation rate of the methanol but not its maximum biodegradation rate. Noncompetitive inhibitors bind at a different site than the substrate. They act by changing the shape of the enzyme, and therefore its activity. The maximum rate is changed but not Km. Uncompetitive inhibitors bind to the enzyme-substrate complex, preventing the formation of products. Both maximum rate and Km are reduced.

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