The equations describing pH effects are therefore analogous to inhibition equations. For single-substrate reactions the pH behavior of the parameters k 0 and k A can sometimes be represented by an equation of the form.
The constants K 1 and K 2 can sometimes be identified as acid dissociation constants for the enzyme. The identification is, however, never straight forward and has to be justified by independent evidence.
It is not accidental that this section has referred exclusively to pH dependences of k 0 and k A. The pH dependence of the initial rate or, worse, the extent of reaction after a given time is rarely meaningful; the pH dependence of the Michaelis constant is often too complex to be readily interpretable.
When using Representative Method As you might expect, this requirement places a serious limitation on kinetic methods of analysis. One solution to this problem is to stop, or quench the reaction by adjusting experimental conditions. For example, many reactions show a strong pH dependency, and may be quenched by adding a strong acid or a strong base. Figure The reaction has a maximum rate at a pH of 5. Increasing the pH by adding NaOH quenches the reaction and converts the colorless p -nitrophenol to the yellow-colored p -nitrophenolate, which absorbs at nm.
What you will have will be this: You no longer have the ability to form ionic bonds between the substrate and the enzyme.
Kinetics The rates of enzyme-catalysed reactions vary with pH and often pass through a maximum as the pH is varied. The pH dependence of the Michaelis constant is often too complex to be readily interpretable.
There is a common misconception that enzymes are destroyed by stomach acid. Nothing could be further from the truth. Stomach acid does not digest protein. Rather, it activates an enzyme called pepsinogen which then becomes pepsin that is secreted by the stomach wall. This enzyme is only active within the pH range of 3.
Pepsin is very specific in its action and is simply incapable of digesting food enzymes, which are very large molecules and are more than just protein. More than seventy years ago, Olaf Bergeim conducted a series of experiments on salivary digestion at the Laboratory of Physiological Chemistry in the University of Illinois, College of Medicine in Chicago. He concluded that a very considerable degree of starch digestion may be brought about by saliva if food is chewed properly.
The pH within the stomach rarely, if ever, drops below 3. The cells that line the stomach -- called parietal cells -- secrete hydrochloric acid or HCl, and this acid gives gastric juices their low pH. HCl does not digest food, but it kills bacteria, helps break down the connective tissue in meat, and activates pepsin, the stomach's digestive enzyme.
Chief cells, which also line the stomach, produce a pro-enzyme called pepsinogen. When pepsinogen contacts the acidic environment of the stomach, it catalyzes a reaction to activate itself and becomes the active enzyme called pepsin.
Pepsin is a protease, or an enzyme that breaks chemical bonds in protein. Pepsin uses the carboxylic acid group on one of its amino acids to break the chemical bond between nitrogen and oxygen in the proteins found in food. The reason pepsin functions best at pH 2 is because the carboxylic acid group on the amino acid in the enzyme's active site must be in its protonated state, meaning bound to a hydrogen atom.
At low pH the carboxylic acid group is protonated, which allows it to catalyze the chemical reaction of breaking chemical bonds. At pH values higher than 2, the carboxylic acid becomes deprotonated and thus unable to participate in chemical reactions.
Pepsin is most active at pH 2, with its activity decreasing at higher pH and dropping off completely at pH 6. In general, enzyme activity is sensitive to pH because the catalytic group of an enzyme -- in pepsin's case, the carboxylic acid group -- will be either protonated or deprotonated, and this state determines whether or not it can participate in a chemical reaction.
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