COMPARATIVE-ANALYSIS OF SPECIES-INDEPENDENT, ISOZYME-SPECIFIC AMINO-ACID SUBSTITUTIONS IN MAMMALIAN MUSCLE, BRAIN AND LIVER-GLYCOGEN PHOSPHORYLASES

Mammalian glycogen phosphorylases exist as three isozymes, muscle, bra in and liver, that exhibit different responses to activation by phosph orylation and AMP, regardless of species. To identify species-independ ent, amino-acid substitutions that may be important determinants in di fferential isozyme control, we have sequenced cDNAs containing the ent ire protein coding regions of rat muscle and brain phosphorylases. Nuc leotide sequence comparisons with rat liver, rabbit muscle, and human muscle, brain and liver phosphorylase genes, indicate that muscle and brain isozymes are more related to each other than to the liver isozym e. Unlike the human isozymes, there is little difference in GC content of codons in the rat isozymes. In relation to the rabbit muscle isozy me three-dimensional structure, amino-acid sequence comparisons indica te that very few nonconservative isozyme-specific substitutions occur in buried and dimer contact residues. There is strict conservation of active site, pyridoxal-phosphate-binding site and nucleoside inhibitor site residues, as well as CAP loop and helix-2 residues that comprise the phosphorylation activation and part of the AMP binding sites. In contrast, five liver isozyme-specific substitutions occur between resi dues 313-325 and another at residue 78 which may be important determin ants in the poor activation of this isozyme by AMP. Substitutions in t he brain isozyme at residues 21-23, 405 and 435 may play a role in its poor response to activation by phosphorylation.