Enzyme Kinetics as an Approach to Understanding Mechanism: -Enzyme Activity Depends on pH

Enzymes have an optimum pH (or pH range) at which their activity is maximal (Fig. 6–17); at higher or lower pH, activity decreases. This is not surprising. Amino acid side chains in the active site may act as weak acids and bases with critical functions that depend on their maintaining a certain state of ionization, and elsewhere in the protein ionized side chains may play an essential role in the interactions that maintain protein structure. Removing a proton from a His residue, for example, might eliminate an ionic interaction essential for stabilizing the active conformation of the enzyme. A less common cause of pH sensitivity is titration of a group on the substrate.

The pH range over which an enzyme undergoes changes in activity can provide a clue to the type of amino acid residue involved (see Table 3–1). A change in activity near pH 7.0, for example, often reflects titration of a His residue. The effects of pH must be interpreted with some caution, however. In the closely packed environment of a protein, the pKa of amino acid side chains can be significantly altered. For example, a nearby positive charge can lower the pKa of a Lys residue, and a nearby negative charge can increase it. Such effects sometimes result in a pKa that is shifted by several pH units from its value in the free amino acid. In the enzyme acetoacetate decarboxylase, for example, one Lys residue has a pKa of 6.6 (compared with 10.5 in free lysine) due to electrostatic effects of nearby positive charges.

FIGURE 6–17 The pH-activity profiles of two enzymes. These curves are constructed from measurements of initial velocities when the re action is carried out in buffers of different pH. Because pH is a logarithmic scale reflecting tenfold changes in [H⁺], the changes in V0are also plotted on a logarithmic scale. The pH optimum for the activity of an enzyme is generally close to the pH of the environment in which the enzyme is normally found. (a)Pepsin, which hydrolyzes certain peptide bonds of proteins during digestion in the stomach, has a pH optimum of about 1.6. The pH of gastric juice is between 1 and 2. (b)Glucose 6-phosphatase of hepatocytes (liver cells), with a pH optimum of about 7.8, is responsible for releasing glucose into the blood. The normal pH of the cytosol of hepatocytes is about 7.2.

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مواضيع ذات صلة

Examples of Enzymatic Reactions:- Hexokinase Undergoes Induced Fit on Substrate Binding

Examples of Enzymatic Reactions:-The Chymotrypsin Mechanism Involves Acylation and Deacylation of a Ser Residue

Enzyme Kinetics as an Approach to Understanding Mechanism: -Enzymes Are Subject to Reversible or Irreversible Inhibition

Enzyme Kinetics as an Approach to Understanding Mechanism: -Pre–Steady State Kinetics Can Provide Evidence for Specific Reaction Steps

Enzyme Kinetics as an Approach to Understanding Mechanism:- Many Enzymes Catalyze Reactions with Two or More Substrates

Enzyme Kinetics as an Approach to Understanding Mechanism: -Kinetic Parameters Are Used to Compare Enzyme Activities

Enzyme Kinetics as an Approach to Understanding Mechanism: -The Relationship between Substrate Concentration and Reaction Rate Can Be Expressed Quantitatively

Enzyme Kinetics as an Approach to Understanding Mechanism: -Substrate Concentration Affects the Rate of Enzyme-Catalyzed Reactions

How Enzymes Work:- Specific Catalytic Groups Contribute to Catalysis

How Enzymes Work:- Binding Energy Contributes to Reaction Specificity and Catalysis

How Enzymes Work:- Weak Interactions between Enzyme and Substrate Are Optimized in the Transition State

Protein Interactions Modulated by Chemical Energy: Actin, Myosin, and Molecular Motors: -Additional Proteins Organize the Thin and Thick Filaments into Ordered Structures