MODULATION OF SYRIAN-HAMSTER 3-HYDROXY-3-METHYLGLUTARYL-COA REDUCTASE-ACTIVITY BY PHOSPHORYLATION - ROLE OF SERINE-871

Attenuation of Syrian hamster 3-hydroxy-3-methyl-glutaryl coenzyme A r eductase (HMG-CoA reductase, EC 1.1.1.34) activity by in vitro phospho rylation was studied using AMP-activated protein kinase and wild type and mutant forms of HMG-CoA reductase. The only residue of the wild-ty pe enzyme phosphorylated was Sers?l. Substrates protected against kina se mediated attenuation of activity, consistent with substrate-induced conformational changes at the C-terminal region. Although close to th e catalytic histidine His(865), Ser(871) appears to play no direct rol e in catalysis or substrate recognition. Mu tent enzymes S871A, S871H, S871N, and S871Q exhibited from 62-106% of wild-type activity and had wild-type K-M values for HMG-CoA and NADPH. Replacement of Ser(871) b y aspartate or glutamate, but not by glutamine, asparagine, histidine, or tyrosine, severely attenuated activity, Attenuation of catalytic a ctivity that accompanies phosphorylation thus appears to result primar ily from the introduction of negative charge, not merely steric hindra nce. Other than the wild-type enzyme, only mutant enzyme S871T was pho sphorylated, and phosphorylation was accompanied by attenuation of act ivity. The AMP-activated kinase thus can also phosphorylate threonyl r esidues.