Glycogen synthase sequences

Figure 18-7. Control of glycogen synthase in muscle (n = number of glucose residues). The sequence of reactions arranged in a cascade causes amplification at each step, allowing only nanomole quantities of hormone to cause major changes in glycogen concentration. (GSK, glycogen synthase kinase-3, -4, and -5 G6P, glucose 6-phosphate.)...

A metabolite acting as an allosteric effector turns on an enzyme that either acts directly on that metabolite or acts on a product that lies further ahead in the sequence. For example, in Fig. 11-1, metabolite C activates the enzyme that catalyzes an essentially irreversible reaction of compound D. An actual example is provided by glycogen synthase, whose inactive "dependent" or D form is activated allosterically by the glycogen precursor glucose 6-phosphate.39 See also phosphorylase kinase (Section 2). 

Table 4.10 Alignment of amino acid sequences of bacterial glycogen synthases and plant starch synthases in the region of the conserved putative loop. Residues with gray background are 100% conserved. The number before the source indicates the number of amino acids of the protein, while the number before the sequence is the first aligned amino acid of each protein. The secondary structure prediction of the E. coli sequence is on top of the alignment E -sheet C loop H helix...

As mentioned above, cAMP-dependent protein kinase phosphorylates and inactivates liver and muscle glycogen synthase [85,86]. Phosphorylation occurs at several serine residues in the muscle enzyme designated la, lb, 2, 3 and 4 [87-90] and it is probable that the phosphorylation pattern is similar in the liver enzyme [91,92]. Phosphorylation of site 1 of the muscle enzyme occurs more rapidly than the other sites [87], and sites 3 and 4 have only recently been recognized as phosphorylation sites [89]. All the sites are surrounded by amino acid sequences characteristic for cAMP-dependent protein kinase [90,93,94]. 

The complete deduced amino-acid sequences of the open reading frames of the Waxy genes from maize and barley are known. About 75% identity can be seen in the sequence with respect to amino acids. If functionally similar amino acids are considered, then the homology is about 81%. Thus, these two proteins are similar in sequence and probably carry out the same function in the starch granule. There is, however, very little sequence homology with the bacterial glycogen synthase beyond the N-terminal sequence. 

Furukawa, K., Tagaya, M., Inouye, M., Preiss, J., and Fukui, T. 1990. Identification of lysine 15 at the active site in Escherichia coli glycogen synthase Conservation of a Lys-X-Gly-Gly sequence in the bacterial and mammalian enzymes. J. BioL Chem. 265,2086-2090.871. 

Figure 21.17 Coordinate control of glycogen metabolism. Glycogen metabolism is regulated, in part, by hormone triggered cyclic AMP cascades. The sequence of reactions leading to the activation of protein kinase A ultimately activates glycogen degradalion. At the same time, protein kinase A also inactivates glycogen synthase, shutting down glycogen synthesis.

The structural gene for the E. coli and S. typbimurium glycogen synthases, glgA, has been cloned ° and the nucleotide sequence of the E. coli glgA gene has been determined. It consists of 1431 bp specifying a protein of 477 amino acids with a molecular weight of 52 412. 

The DNA sequence of the Waxy locus of Zea mays was determined by analysis of both a genomic and an almost full-length cDNA clone.Also, barley Waxy locus has been cloned and its DNA sequenced.The deduced amino acid sequences of the maize and barley clones can be compared with the amino acid sequence of the E. coli ADP-Glc-specific glycogen synthase. Both clones had the sequence seen in the bacterial enzyme... 

This interdependency of GSK-3 with casein kinase II has been defined as hierarchal phosphorylation. Another example of the hierarchal phosphorylation is seen with cyclic-AMP-dependent protein kinase and casein kinase I. The cAMP-dependent protein kinase enhances phosphorylation of the glycogen synthase by casein kinase The phosphorylation by casein kinase I was serine residue Synthetic peptides based on the four phosphoryiated regions in the muscle glycogen sequence (residues 694—707, 706-733, 1-14, and 636-662) were synthesized and phosphoryiated. Casein kinase I could not phosphorylate the unphosphorylated peptides but, if cAMP-dependent kinase phosphoryiated peptides, 694—707, 706-733, and 1-14, all three peptides were easily phosphoryiated by the casein kinase I. The greatest stimulation was seen with peptide 1-14. In the case of peptide... 

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