Phosphorylation contraction cycle

Although the fundamental chemomechanical transduction processes seem to be the same in all types of vertebrate muscle, contraction in smooth muscle is characterized by much greater involvement of enzymatically catalyzed control reactions. In smooth muscle the control reactions themselves involve the use of phosphorylation-dephosphorylation cycles. Moreover, they are futile in the sense they cause the expenditure of bond energy without a tangible work resultant, i.e., compounds synthesized or external work done. 

When smooth muscle myosin is bound to F-actin in the absence of other muscle proteins such as tropomyosin, there is no detectable ATPase activity. This absence of activity is quite unlike the situation described for striated muscle myosin and F-actin, which has abundant ATPase activity. Smooth muscle myosin contains fight chains that prevent the binding of the myosin head to F-actin they must be phosphorylated before they allow F-actin to activate myosin ATPase. The ATPase activity then attained hydrolyzes ATP about tenfold more slowly than the corresponding activity in skeletal muscle. The phosphate on the myosin fight chains may form a chelate with the Ca bound to the tropomyosin-TpC-actin complex, leading to an increased rate of formation of cross-bridges between the myosin heads and actin. The phosphorylation of fight chains initiates the attachment-detachment contraction cycle of smooth muscle. 

The calmodulin-4Ca +-activated light chain kinase phosphorylates the hght chains, which then ceases to inhibit the myosin-F-actin intetaction. The contraction cycle then begins. 

The control of glycogen phosphorylase by the phosphorylation-dephosphorylation cycle was discovered in 1955 by Edmond Fischer and Edwin Krebs50 and was at first regarded as peculiar to glycogen breakdown. However, it is now abundantly clear that similar reactions control most aspects of metabolism.51 Phosphorylation of proteins is involved in control of carbohydrate, lipid, and amino acid metabolism in control of muscular contraction, regulation of photosynthesis in plants,52 transcription of genes,51 protein syntheses,53 and cell division and in mediating most effects of hormones. 

In principle, one can classify the phosphoproteins into two groups (1) functional, whose phosphorylation is correlated with contraction, and (2) structural, whose phosphate content remains rather steady during the contraction cycle. Structural phosphoproteins could make contact with other proteins to form a specific protein network. Alternatively, they may bind divalent metals, Ca + or Mg2+. The common experience of the slow turnover of phosphate in these proteins also suggests that the covalently bound phosphate is not free and, therefore, not readily available for protein kinases and phosphatases. Future investigation should provide information about the role of the structural phosphoproteins in smooth muscle. 

Ryanodine receptors are a family of intracellular Ca release channels that were originally identified in the sarcoplasmic reticulum of skeletal muscle cells. Three members of the family were distinguished, RyR2 ryanodine receptors in the cardiac muscle. RyRl (in the skeletal muscle) and RyR2 function as Ca release chaimels from the sarcoplasmic reticulum intracellular calcium store and play a crucial role in the exdtation-contraction cycle. They bind and calmodulin and become phosphorylated by various protein kinases including Ca /calmodulin- and cAMP-dependent kinases (Lokuta etal. 1995, Mayrleitner etal. 1995). 

The ATP required as the constant energy source for the contraction-relaxation cycle of muscle can be generated (1) by glycolysis, using blood glucose or muscle glycogen, (2) by oxidative phosphorylation, (3) from creatine... 

The simultaneous activities of the kinase and phosphatase are yet another example of regnlation by reversible protein phosphorylation (i.e. an interconversion cycle -Chapter 3). An increased force of contraction could be caused either by inhibition of the phosphatase or by activation of the kinase. However, physiologically relevant inhibitors of the phosphatase have not yet been discovered. 

PPI is a major dass of eukaryotic Ser/Thr-specific protein phosphatases that regulate diverse cellular processes such as cell cycle progression, muscle contraction, carbohydrate metabolism, protein synthesis, transcription, and neuronal signaling. Its action is modulated and regulated by assodation with subunits induding various inhibitor proteins and multiple targeting subunits of which nearly 30 proteins have now been identified (review Aggen et al., 2000). The activity of the inhibitory proteins can be controlled via phosphorylation by protein kinase A as outlined in Fig. 7.16. 

The exchange of the covalently bound phosphate of LC20 during contraction (see Section V) supports a relationship between LC20 phosphorylation and cross-bridge cycling rate. 

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