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

We have introduced kinetics as the primary method for studying the steps in an enzymatic reaction, and we have also outlined the limitations of the most common kinetic parameters in providing such information. The two most important experimental parameters obtained from steady-state kinetics are kcat and k_cat/K_m. Variation in k_cat and k_cat/K_m with changes in pH or temperature can provide additional information about steps in a reaction pathway. In the case of bisubstrate reactions, steady state kinetics can help determine whether a ternary complex is formed during the reaction (Fig. 6–14). A more complete picture generally requires more sophisticated kinetic methods that go beyond the scope of an introductory text. Here, we briefly introduce one of the most important kinetic approaches for studying reaction mechanisms, pre–steady state kinetics. A complete description of an enzyme-catalyzed re action requires direct measurement of the rates of in dividual reaction steps—for example, measurement of the association of enzyme and substrate to form the ES complex. It is during the pre–steady state that the rates of many reaction steps can be measured independently. Experimenters adjust reaction conditions so that they can observe events during reaction of a single substrate molecule. Because the pre–steady state phase is generally very short, the experiments often require special ized techniques for very rapid mixing and sampling. One objective is to gain a complete and quantitative picture of the energy changes during the reaction. As we have already noted, reaction rates and equilibria are related to the free-energy changes during a reaction. Measuring the rate of individual reaction steps reveals how energy is used by a specific enzyme, which is an important component of the overall reaction mechanism. In a number of cases investigators have been able to record the rates of every individual step in a multistep enzymatic reaction. Some examples of the application of pre–steady state kinetics are included in the descriptions of specific enzymes in Section 6.4.

FIGURE 6–14 Steady-state kinetic analysis of bisubstrate reactions. In these double-reciprocal plots (see Box 6–1), the concentration of substrate 1 is varied while the concentration of substrate 2 is held con stant. This is repeated for several values of [S2], generating several separate lines. (a) Intersecting lines indicate that a ternary complex is formed in the reaction; (b) parallel lines indicate a Ping-Pong (double-displacement) pathway.

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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: -Enzyme Activity Depends on pH

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

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