Structure of Phosphoprotein Metabolically Active in Phosphorylation and Dephosphorylation Reactions

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Abstract
The susceptibilities of phosphoproteins to protein kinase [EC 2.7.1.37] and phosphoprotein phosphatase [EC 3.1.3.16] were studied using samples of ling and salmon phosvitins of various phosphorus contents. Phosphorylation was maximal at a phosphorus content characteristic of each species of phosvitin, i.e., 6.7%, 7.6% and 8.7% for hen, salmon and ling phosvitins, respectively. Dephosphorylation, however, did not increase to a maximum on increasing the phosphorus content but gave a sigmoidal curve indicating that enzyme activity depends on the state of the phosphate in the phosvitin. This was supported by analysis and comparison of the products of the phosphatase reaction. Selective phosphorylation and dephosphorylation were concluded to occur in blocks of phosphoserine residues from analysis of phosphopeptides formed by partial hydrolysis of the products in the two reactions. It was thus concluded that the high phosphate turnover in the phosphoprotein molecule is due to active phosphorylation and dephosphorylation of blocks of phosphoserine residues. Comparison of the secondary structure of salmon phosvitin with its activity in phosphorylation indicated that phosvitin may have three types of structure depending on its phosphorus content, i.e., a cross-β structure with a low phosphorus content, a parallel-β structure with a high phosphorus content, and at intermediate phosphorus contents a random coil structure, which forms an a-helical structure at pH values below the isoelectric point. This last structure is thought to be the active form acting as acceptor in phosphorylation. The contribution of the structure to susceptibility to enzymes is discussed.

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