Interconvertible enzyme

FIGURE 15.2 Enzymes regulated by covalent modification are called interconvertible enzymes. The enzymes protein kinase and protein phosphatase, in the example shown here) catalyzing the conversion of the interconvertible enzyme between its two forms are called converter enzymes. In this example, the free enzyme form is catalytically active, whereas the phosphoryl-enzyme form represents an inactive state. The —OH on the interconvertible enzyme represents an —OH group on a specific amino acid side chain in the protein (for example, a particular Ser residue) capable of accepting the phosphoryl group. 

Cyclic cascades can achieve great signal amplification, and interconvertible enzymes can respond to effectors at concentrations considerably below their respective dissociation constants. 

A parameter used to assist in the characterization of enzyme cascade systems. Symbolized by S A, it is equal to [eo.5E]/[eo.5i] where [eo.sE] is the concentration of effector required for 0.5 activation of the converter enzyme E and [eo.5i] is the concentration of effector at which 50% of the interconvertible enzyme (1) has been modified See Enzyme Cascade Kinetics P. B. Chock E. R. Stadtman (1980) Meth. Enzymol. 64, 297. 

B. P. Chock and E. R Stadtman. Covalently interconvertible enzyme cascade systems. Meih EmymoL, 64, 397 25, 1980. 

S.G Rhee, R. Park, P.B. Chock, and E.R. Stadtman. 1978. Allosteric regulation of monocyclic interconvertible enzyme cascade systems Use of Escherichia coli glutamine synthetase as an experimental model Proc. Natl. Acad. Sci. USA 75 3138-3142. (PubMed)... 

Rhee, S. G., Park, R,. Chock, P. B.. and Sladlman, E. K. 197 S. Allo.steric regulation of monocyclic interconvertible enzyme as-cade systems Use of Escherichui co/i glutamine synthetase asancx perimental model. Proc. Natl Acad. Sci. U. S. A. 75 3138. 1142. 

At this juncture we would like to remind the reader that HMGR is an interconvertible enzyme, which exists in phosphorylated (inactive) and dephosphorylated (active) forms. The occurrence of these two forms is at the center of the activity of the enzyme. The active inactive interconversion involves a protein Kinase cascade which operates via cyclic AMP (cAMP) as a secondary messenger [26]. 

Interconvertible Enzyme Cascades in Metabolic Regulation E. R. Stadtman and P. B. Chock... 

From our experiences in the aflSnity labeling of steroid interconverting enzymes we offer the following suggestions ... 

The catalystic component, r , affects both forward and reverse processes of a reaction (Section III,B). If interconvertible enzyme forms are present, their effects are incorporated into r (Appendix E). 

There are many examples, including glycogen phosphorylase, glycogen synthetase, pyruvate dehydrogenase (35), and the diflFerence from a classical interaction is that regulator X interacts with an interconverting enzyme (Ep) instead of the pathway enzyme (a or b). 

This function shows that, for any given interaction between X and Ep (i.e., s x))> the response of [a] to X is greater the smaller the values of and s 5b) Consequently, for a sensitive response of the pathway enzyme to X, the sensitivities of the interconverting enzymes to the a and b forms must be low. Low sensitivites can be produced by a near-saturation of the interconverting enzymes with the a and b forms and this has been termed zero-order ultrasensitivity by Koshland and his associates (16, 29). 

Stadtman, E. R., and Chock, P. B., 1977, Superiority of interconvertible enzyme cascades in metabolic regulation Analysis of monocyclic systems, Proc. Natl. Acad. Sci. USA 74 2761. 

Steinberg, D., 1976, Interconvertible enzymes in adipose tissue regulated by cyclic AMP-dependent protein kinase, Adv. Cyclic Nucleotide Res. 7 157. 

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