Zero-order ultrasensitivity in the regulation of glycogen phosphorylase.

Abstract

The activity of glycogen phosphorylase (1,4-a-D-glucan:orthophosphate a-D-glucosyltransferase, EC 2.4.1.1) is controlled by a cyclic phosphorylation-dephosphorylation process through the action of the interconverting enzymes, phosphorylase b kinase (ATP:phosphorylase-b phosphotransferase, EC 2.7.1.38) and phosphorylase a phosphatase (phosphorylase a phosphohydrolase, EC 3.1.3.17). In muscle tissue, the combined concentration of the activated (phospho-) form, phosphorylase a, and the nonactivated (dephospho-) form, phosphorylase b, is substantially greater than the Km of either of the interconverting enzymes for its phosphorylase substrate. It has been predicted that, under such a set of conditions, a sensitivity amplification will occur for phosphorylase regulation due to the zero-order ultrasensitivity effect [LaPorte, D. C. & Koshland, D. E., Jr. (1983) Nature (London) 305, 286-290]. The sensitivity amplification will enhance the responsiveness of the phosphorylase interconversion cycle to changes in the ratio of activities of the kinase to phosphatase. We have studied the cyclic interconversion process using purified muscle enzymes in steady-state reactions and found that there is an enhancement in the control sensitivity of the process due to the zero-order ultrasensitivity effect. The potential for the in vivo enhancement of sensitivity in glycogen degradation by this effect is discussed.The production of glucose 1-phosphate from glycogen and inorganic phosphate is controlled by the activity of glycogen phosphorylase (1,4-a-D-glucan:orthophosphate a-D-glucosyltransferase, EC 2.4.1.1), an enzyme regulated by the welldescribed phosphorylation/dephosphorylation cycle shown in Fig. 1 (1-3). The sequence of hormone-k adenylate cyclase --cAMP --cAMP-dependent protein kinase --phosphorylase b kinase -* phosphorylase --glucose 1-phosphate can be estimated to produce a potential signal amplification with a magnitude change of 106 even after dilution and other losses. The large magnitude amplification § of such cascades will often be unnecessary in physiologically well-regulated systems, where homeostasis will allow only relatively small changes in metabolite levels. In contrast to magnitude amplification, sensitivity amplification processes are those in which small relative changes in the concentration of an effector can result in a larger relative change in the response (for example, a 2-fold change in effector concentration might produce a 10-fold increase in the response). Three methods of sensitivity amplification have been compared by Koshland et al. (4): (i) positive cooperativity, (it) multiple inputs along a pathway, and (iii) the recently described zero-order ultrasensitivity (5). The enhanced sensitivity in this latter case occurs in cyclic activation systems when one or both of the interconverting enzymes are nearly saturated with substrate and, therefore, are operating in or near their "zeroorder" region with regard to substrate. (inactive) is catalyzed by the enzymes phosphorylase b kinase (ATP...

Artificial Biochemistry

“…On that basis, we now reproduce the peculiar phenomenon of hypersensitivity in zero-order regime [7], confirming that our simplified kinetics reproduces effects of enzyme kinetics. In a hypersensitivity situation, a minor switch in relative enzyme quantities, creates a much amplified and sudden switch in two other quantities.…”

Activation of the Liver Glycogen Phosphorylase by Ca2+Oscillations: a Theoretical Study

“…This result is a direct consequence of the saturation of the converter enzymes by their substrates, leading to a phenomenon known as &&zero-order ultrasensitivity'' (Goldbeter & Koshland, 1981), and of the cooperativity in the kinase activation by Ca> (Dupont & Goldbeter, 1992). Interestingly, in the case of muscle glycogen phosphorylase (Meinke et al, 1986), &&zero-order ultrasensitivity'' has indeed been demonstrated experimentally.…”

Phosphatase Specificity and Pathway Insulation in Signaling Networks

“…For one, none of the substrates can respond in a switch-like manner to incoming signals, so this mechanism cannot be deployed in cases where ultrasensitive responses are crucial (11,16,(44)(45)(46)(47). In addition, achieving fast dephosphorylation timescales may require high levels of phosphatase expression, which may become impractical (or limit the capacity of the system to respond at all) in cases where the K M needed to achieve insulation is very large (Figs.…”