In skeletal muscle the activation of phosphorylase b is catalyzed by phosphorylase kinase. Both enzymes occur in vivo as part of a multienzyme complex. The two enzymes have been imaged by atomic force microscopy and the results compared to those previously found by scanning tunneling microscopy. Scanning tunneling microscopy and atomic force microscopy have been used to view complexes between the activating enzyme phosphorylase kinase and its substrate phosphorylase b. Changes in the size and shape of phosphorylase kinase were observed when it bound phosphorylase b.

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...

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