Effect of pH

Like all proteins, enzymes have a variety of acidic and basic groups, each with its own ionization constant. For the enzyme to have optimum activity, it must exist in a particular ionization state. If the pH changes enough to change the ionization significantly on any one group, the enzyme's activity will decrease. This can be modeled by assuming that the enzyme is in equilibrium with two versions of itself, one containing one additional proton over the optimum, E+, and another with one proton less than optimum, E-, with equilibrium constants associated with the conversion to each of those states:

With this mechanism it is possible to derive the following equation for the fraction of the total enzyme in the optimum state as a function of the hydrogen ion concentration:

Notice that this has the same form as the Haldane equation for substrate inhibition as in equation (5.38). In a similar fashion, the optimum pH is found to be pHoptimum 2 (5:45)

Figure 5.3 shows the effect of pH on enzyme activity according to equation (5.44). All curves have an optimum pH of 7.0. The outer curve has pK1 and pK2 values of 4 and 10, respectively; inner curves are 5 and 9, 6 and 8, and 6.75 and 7.25, respectively. Note that the farther apart the pKfl values, the broader the optimum. Furthermore, if the pKfl values are close to each other, the maximum activity falls substantially below 100%. This means

Figure 5.3 shows the effect of pH on enzyme activity according to equation (5.44). All curves have an optimum pH of 7.0. The outer curve has pK1 and pK2 values of 4 and 10, respectively; inner curves are 5 and 9, 6 and 8, and 6.75 and 7.25, respectively. Note that the farther apart the pKfl values, the broader the optimum. Furthermore, if the pKfl values are close to each other, the maximum activity falls substantially below 100%. This means

that even at the optimum pH, much of the enzyme is in an inactive form because one or the other of the acid-base groups is in the wrong state of association.

Was this article helpful?

0 0
Quit Smoking Today

Quit Smoking Today

Quit smoking for good! Stop your bad habits for good, learn to cope with the addiction of cigarettes and how to curb cravings and begin a new life. You will never again have to leave a meeting and find a place outside to smoke, losing valuable time. This is the key to your freedom from addiction, take the first step!

Get My Free Ebook


Post a comment