Ferrer A, Caelles C, Massot N, Hegardt F G
Biol Chem Hoppe Seyler. 1987 Mar;368(3):249-57. doi: 10.1515/bchm3.1987.368.1.249.
Microsomal 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase kinase activity is enhanced about 5 fold by 2 mM of either AMP or ADP. Activation constants, Ka, for AMP and ADP are 17 microM and 430 microM respectively, showing that AMP is a more potent activator than ADP. This property is expressed by increasing not only the rate of reductase inactivation but also the rate of reductase phosphorylation from [gamma-32P]ATP. GTP can replace ATP as substrate of reductase kinase but GMP and GDP cannot replace AMP as activators. Kinetic studies show that ATP can only act as a substrate. Nucleoside mono or diphosphates and nucleoside triphosphates, thus, appear to bind to different sites on microsomal HMG-CoA reductase kinase. Nucleoside mono or diphosphates act as allosteric activators of reductase kinase. The adenosyl moiety and the unaltered phosphate ester at the 5' position are two essential features of the activator molecule. Phosphorylation of reductase either by microsomal or cytosolic AMP-activated reductase kinase produces an 80% inactivation, with a concomitant incorporation of 0.8 mol of 32P per mol of reductase (Mr 55,000). In both cases exhaustive tryptic digestion of 32P-labeled HMG-CoA reductase, which had been denatured in 2M urea, yields two major phosphopeptides, the phosphoryl group being bound to serine residues.
微粒体3-羟基-3-甲基戊二酰辅酶A（HMG-CoA）还原酶激酶活性可被2 mM的AMP或ADP增强约5倍。AMP和ADP的激活常数Ka分别为17 μM和430 μM，表明AMP是比ADP更有效的激活剂。这种特性不仅通过增加还原酶失活速率来体现，还通过增加还原酶从[γ-32P]ATP的磷酸化速率来体现。GTP可以替代ATP作为还原酶激酶的底物，但GMP和GDP不能替代AMP作为激活剂。动力学研究表明ATP只能作为底物。因此，核苷单磷酸或二磷酸以及核苷三磷酸似乎结合在微粒体HMG-CoA还原酶激酶的不同位点上。核苷单磷酸或二磷酸作为还原酶激酶的变构激活剂。激活剂分子的两个基本特征是腺苷部分和5'位未改变的磷酸酯。微粒体或胞质AMP激活的还原酶激酶对还原酶的磷酸化都会导致80%的失活，同时每摩尔还原酶（Mr 55,000）会掺入0.8摩尔的32P。在这两种情况下，对在2M尿素中变性的32P标记的HMG-CoA还原酶进行彻底的胰蛋白酶消化，会产生两种主要的磷酸肽，磷酸基团与丝氨酸残基结合。
