Enzyme reversible changes

A quite different approach came from Chance and others using heme enzymes (1947). Purified horseradish peroxidase has a characteristic absorption spectrum which was visibly altered in the presence of hydrogen peroxide. When an appropriate substrate was added it was oxidized by the hydrogen peroxide and the spectrum reverted to that of the original state of the enzyme. Similar studies were performed with catalase, showing that prosthetic groups in enzymes underwent reversible changes in the course of their reactions. 

Figure 20.18 The central dogma of molecular biology a summary of processes involved inflow of genetic information from DNA to protein. The diagram is a summary of the biochemical processes involved in the flow of genetic information from DNA to protein via RNA intermediates. This concept had to be revised following the discovery of the enzyme, reverse transcriptase, which catalyses information transfer from RNA to DNA (see Chapter 18). It may have to be modified in the future since changes in the fatty acid composition of phospholipids in membranes can modily the properties of proteins, and possibly their functions, independent of the genetic information within the amino acid sequence of the protein (See Chapters 7, 11 and 14).

Metal Ion Catalysis Metals, whether tightly bound to the enzyme or taken up from solution along with the substrate, can participate in catalysis in several ways. Ionic interactions between an enzyme-bound metal and a substrate can help orient the substrate for reaction or stabilize charged reaction transition states. This use of weak bonding interactions between metal and substrate is similar to some of the uses of enzyme-substrate binding energy described earlier. Metals can also mediate oxidation-reduction reactions by reversible changes in the metal ion s oxidation state. Nearly a third of all known enzymes require one or more metal ions for catalytic activity. 

Intact AChE is a basic requirement for normal function of the organism and thus for effective prophylaxis. The enzyme is changed in a way that will make it resistant to OP. This can be achieved by using reversible inhibitors (preferably carbamates), which are able to inhibit AChE reversibly, and after spontaneous recovery of the activity (decarbamylation) normal AChE serves as a source of an active enzyme. 

Measurements of the temperature dependence of enzymatic activity suggested the presence of an intermediate step in enzyme inactivation. The proposed mechanism involves a reversible change to an inactive state, which precedes the final irreversible inactivation step [21-23]. This mechanism could now be refined based on experiments with single enzymes, which detected the intermediate steps directly. This more detailed information allowed the establishment of a tentative model for a-chymotrypsin inactivation (Fig. 25.2d). 

Noncompetitive inhibitors interact reversibly with enzymes to form an inactive species, effectively removing active enzyme and thus interfering with the rate of conversion of substrate to product. The inhibitor may interact with free enzyme, or with the enzyme-substrate complex. The key feature of noncompetitive inhibition that distinguishes it from competitive inhibition is that inhibition does not affect the apparent affinity of the enzyme for its substrate (i.e., the apparent Km). For example, a noncompetitive inhibitor may bind in a region remote from the active site to cause a reversible change in enzyme tertiary structure that completely prevents substrate binding and product formation. In this type of inhibition, the quantity of active enzyme appears to decrease as inhibitor concentration increases, so that the apparent Fmax for the reaction decreases. 

The body regulates enzyme activity in many ways. It changes the amount of enzyme protein present by changing the rate of synthesis or degradation. In other cases, a covalent modification of the enzyme protein will cause the activity to increase or decrease. Alternatively, other molecules may bind reversibly to the enzyme to change its activity. We will discuss examples of each type of enzyme regulation as they occur in the body. 

This includes both reversible changes like phosphorylation of specific serines in the enzymes of glycogen metabolism and irreversible changes like zymogen activation by proteolysis in digestion and blood clotting. These mechanisms are considered in more detail below. 

Allosteric interactions control the behavior of proteins through reversible changes in quaternary structure, but this mechanism, effective though it may be, is not the only one available. A zymogen, an inactive precursor of an enzyme, can be irreversibly transformed into an active enzyme by cleavage of covalent bonds. 

---