Activators of yeast hexokinase.
نویسندگان
چکیده
The rate of the ATP:glucose transphosphorylase reaction catalyzed by yeast hexokinase, isozyme PII, is inhibited by lowering the pH of the reaction below 7.0, especially at suboptimal concentrations of ATP. This effect of acidity is largely overcome by activators such as orthophosphate, citrate, malate, 3-phosphoglycerate, and riboside triphosphates. Thus, in the acid range, ATP appears to serve both as an activator and a substrate with the result that l/v versus l/[ATP] plots are nonlinear. It appears that yeast hexokinase may exist as two conformational isomers, an inactive form which is favored in the acid range and an active form favored by various polyanions or by alkaline pH. The conversion of inactive to active enzyme by citrate is slow, requiring about 1 min at 25” when citrate is added to the reaction mixture after the substrates. When citrate is added to the enzyme simultaneously with both substrates, the reaction begins at the fully activated rate indicating that only the ternary complex can undergo the conversion to inactive enzyme. Equilibrium isotope exchange experiments indicate that citrate activates the glucose & glucose 6phosphate, ADP ti ATP, and glucose-6-P G ATP exchanges equally. This is consistent with the idea that the ternary complex is undergoing the allosteric interconversions. Native hexokinase assayed inside semipermeable yeast cells exhibits the same kinetic properties as the isolated native enzyme, indicating that the activation phenomena may be important in controlling the rate of hexokinase in vivo.
منابع مشابه
Activation of Yeast Hexokinase PI1
Activation of yeast hexokinase PI1 at low pH by citrate or ATP has been shown to occur at the monomer level, without requiring association to the dimer level. Thus, the previous suggestion (Steitz, T. A., Anderson, W. F., Fletterick, R. G., and Anderson, C. M. (1977) J. Biol. Chem. 252, 4494-4500) that anionic activators of hexokinase, such as citrate and ATP, act by binding to the intersubunit...
متن کاملA slow transient kinetic process of yeast hexokinase.
& Rose (1970) have suggested the possibility of negative co-operativity in yeast hexokinase (PI,), an enzyme that appears to be identical with yeast hexokinase B. In view of the increasing prevalence of transient kinetics and the possible significance of such behaviour in co-operativity, we now report experiments documenting a slow change in hexokinase activity during assay and suggest a unique...
متن کاملThe high resolution crystal structure of yeast hexokinase PII with the correct primary sequence provides new insights into its mechanism of action.
Hexokinase is the first enzyme in the glycolytic pathway, catalyzing the transfer of a phosphoryl group from ATP to glucose to form glucose 6-phosphate and ADP. Two yeast hexokinase isozymes are known, namely PI and PII. The crystal structure of yeast hexokinase PII from Saccharomyces cerevisiae without substrate or competitive inhibitor is determined and refined in a tetragonal crystal form at...
متن کاملCloning of genes that complement yeast hexokinase and glucokinase mutants.
Genes complementing the glucose-negative fructose-negative Saccharomyces cerevisiae triple mutant strain (hxkl hxk2 glk1), which lacks hexokinase PI, hexokinase PII, and glucokinase, were obtained from a pool of yeast DNA in the multicopy plasmid YEp13.
متن کاملذخیره در منابع من
با ذخیره ی این منبع در منابع من، دسترسی به آن را برای استفاده های بعدی آسان تر کنید
عنوان ژورنال:
- The Journal of biological chemistry
دوره 246 8 شماره
صفحات -
تاریخ انتشار 1971