The sulfhydryl groups of muscle phosphorylase. V. the reactive sulfhydryl peptides
Zarkadas, C.G.; Smillie, L.B.; Madsen, N.B.
Canadian Journal of Biochemistry 48(7): 763-776
ISSN/ISBN: 0008-4018 PMID: 4934391 DOI: 10.1139/o70-120
Skeletal muscle glycogen phosphorylase a and b have previously been shown to consist of four and two subunits, respectively, each having a molecular weight of 92 500. Our studies have indicated that there is a minimum of eight and a maximum of nine unique cysteine sequences in the enzyme providing additional evidence that the subunits are identical. From the known sequences of the nine sulfhydryl peptides it was possible to isolate and characterize the reactive sulfhydryl peptides of phosphorylase. Careful column chromatographic purification of alkylated peptides derived from peptic digestion showed that there are two cysteine residues per monomer of phosphorylase b whose rate of reaction with iodoacetamide approaches that of model compounds and whose alkylation does not affect significantly the enzymic properties of the protein. These two cysteines have been identified in sequences corresponding to numbers 2 and 5 previously elucidated. These sequences have been extended in the present work and may now be written as: No. 2 Asn–Gln–Lys–Ile–CMC–Gly–Gly–Try–Gln–Ser, and No. 5 Gly–CMC–Arg–Asp–Pro–Val–Arg–Thr–Asn–Phe.