Protein phosphorylation and the control of glycogen metabolism in skeletal muscle

Abstract

Glycogen metabolism in mammalian skeletal muscle is controlled by a regulatory network in which six protein kinases, four protein phosphatases and several thermostable regulatory proteins determine the activation state of glycogen phosphorylase and glycogen synthase, the rate-limiting enzymes of this process. Thirteen phosphorylation sites are involved, twelve of which have been isolated and sequenced and shown to be phosphorylated in vivo . The effects of adrenalin and insulin on the state of phosphorylation of each site have been determined. The neural control of glycogen metabolism is mediated by calcium ions and involves phosphorylase kinase, and a specific calmodulin-dependent glycogen synthase kinase. The β-adrenergic control of the system is mediated by cyclic AMP, and involves the phosphorylation of phosphorylase kinase, glycogen synthase and inhibitor 1 by cyclic-AMP-dependent protein kinase. Inhibitor 1 is a specific inhibitor of protein phosphatase 1, the major phosphatase involved in the control of glycogen metabolism. The stimulation of glycogen synthesis by insulin results from the dephosphorylation of glycogen synthase at sites (3 a + 3 b + 3 c ), which are introduced by the enzyme glycogen synthase kinase 3. The structure, regulation and substrate specificities of the protein phosphatases involved in glycogen metabolism are reviewed. Protein phosphatase 1 can exist in an inactive form termed the Mg-ATP-dependent protein phosphatase, which consists of a complex between the catalytic subunit and a thermostable protein termed inhibitor 2. Activation of this complex is catalysed by glycogen synthase kinase 3. It involves the phosphorylation of inhibitor 2 and its dissociation from the catalytic subunit. Protein phosphatase 2A can be resolved into three forms by ion exchange chromatography. These species contain the same catalytic subunit and other subunits that may have a regulatory function. Protein phosphatase 2B is a Ca 2+ -dependent enzyme composed of two subunits, A and B. Its activity is increased tenfold by calmodulin, which interacts with the A-subunit. The B-subunit is a Ca 2+ -binding protein that is homologous with calmodulin. Its N-terminus contains the unusual myristyl blocking group, only found previously in the catalytic subunit of cyclic-AMP-dependent protein kinase. Protein phosphatase 2C is a Mg 2+ -dependent enzyme that accounts for a very small proportion of the glycogen synthase phosphatase activity in skeletal muscle. It is likely to be involved in the regulation of other metabolic processes in vivo such as cholesterol synthesis. Recent evidence suggests that many of the proteins involved in the control of glycogen metabolism have much wider roles, and that they participate in the neural and hormonal regulation of a variety of intracellular processes.

Expression of transforming growth factor β-like molecules in normal and regenerating arms of the crinoidAntedon mediterranea: immunocytochemical and biochemical evidence

“…The Western blot (from the 2D gel) performed on material from non-regenerating arms further suggested that this TGF-β-like molecule might exist in at least two isoforms. The close proximity of the isoforms to each other implies that they could result from a single posttranslational modification such as phosphorylation (Cohen 1983;Massagué 1998). During regeneration, significant changes in this profile are evident.…”

Propriétés des fibres musculaires squelettiques. 1. Influence de l'innervation motrice

“…En effet, parmi les méthodes biochimiques permettant d'étudier le métabolisme musculaire au cours de la contraction, celles se rapportant soit à la détermination du quotient respiratoire (rapport C0 2 libéré/0 2 absorbé), soit aux modifications des substrats endogènes ou à l'utilisation des substrats exogènes in vivo et in vitro ont montré leurs limites techniques : difficulté des mesures de substrats spécifiques (entre différents glucides par exemple), difficulté de quantifier des substrats phosphorylés ou non, catabolisés rapidement au cours du prélèvement de l'échantillon musculaire (Corsi et al, 1969). Les Stetten, 1960 ;Cohen, 1983). Toutefois, des études plus récentes montrent que la glycogénolyse n'aurait un rôle important qu'au début de l'activité physique en produisant un surplus d'ATP, ou lorsque l'énergie néces-saire à l'accomplissement de travaux musculaires intenses excède celle produite par l'utilisation des acides gras et autres substrats circulants (Rowell etal., 1966 ;Bergstrôm et al, 1967 ;Havel, 1970 ;Beatty et al, 1972 ;Newsholme et al, 1978).…”

Impaired insulin-stimulated muscle glycogen synthase activation in vivo in man is related to low fasting glycogen synthase phosphatase activity.

“…The dephosphorylated form (I form) is spontaneously active and virtually independent from the allosteric activation by glucose-6-phosphate (G6P);' the phosphorylated form (D form) is inactive and sensitive to the action of the allosteric activator, G6P. The change from the dephosphorylated to the phosphorylated state is carried out by several protein kinases (5,6); the inverse reaction is catalyzed by glycogen synthase phosphatase (protein phosphatase) (7). Insulin enhances glycogen synthase activity by a mechanism involving the activation of glycogen synthase phosphatase (8)(9)(10) and the inhibition of cAMP-dependent protein kinase (11,12).…”

“…Glycogen synthase is the rate-limiting enzyme for glycogen synthesis (4). Its activity can be modulated by allosteric or covalent (phosphorylation or dephosphorylation) modifications of the molecule (5,6). The dephosphorylated form (I form) is spontaneously active and virtually independent from the allosteric activation by glucose-6-phosphate (G6P);' the phosphorylated form (D form) is inactive and sensitive to the action of the allosteric activator, G6P.…”