Structure, Function and Regulation of Protein Phosphatase
In the opening lecture, the events surrounding the discovery that glycogen phosphorylase activity is regulated by phosphorylation-dephosphorylation were presented. Since that time, this type of protein modification has proved to be a major mechanism by which many eukaryotic cellular processes, including their metabolism, growth, differentiation, transformation and gene expression, can be regulated. We have already heard about the many advances that have been made in our understanding of the subunit structure, mechanism of action, functional sites and regulation of the cAMP-dependent protein kinase, of phosphorylase kinase, and several other kinases, including protein tyrosine kinases. Until recently, however, much less was known about the protein phosphatases that catalyse protein dephosphorylation. Reasons for this include: 1) difficulty in obtaining substrates since an appropriate phosphoprotein must first be isolated and then phosphorylated with a specific protein kinase, and 2) general instability, low intracellular concentrations, and broad specificity of the phosphatases in addition to their tendency to be affected by many allosteric agents. This has made it difficult to determine if one were working with a single phosphatase or a mixture of enzymes. Despite these problems, several phosphatases have now been purified and extensively studied and some of their properties will be discussed.
KeywordsProtein Phosphatase Catalytic Subunit Regulatory Subunit Phosphorylase Kinase Specific Protein Kinase
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