Jw. Hudson et al., COMPARATIVE-ANALYSIS OF SPECIES-INDEPENDENT, ISOZYME-SPECIFIC AMINO-ACID SUBSTITUTIONS IN MAMMALIAN MUSCLE, BRAIN AND LIVER-GLYCOGEN PHOSPHORYLASES, Biochimica et biophysica acta, 1164(2), 1993, pp. 197-208
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.