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    188宝金博页面版: Physiological role of glucose-phosphorylating enzymes in Saccharomyces cerevisiae

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    内容提示： ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS 182, 639-645 (1977) Physiological Role of Glucose-Phosphorylating Enzymes in Saccharomyces cerevisiae ZITA LOB0 AND P. K. MAITKA Tata Institute of Fundamental Research, Bombay 400005, India Received January 13, 1977 Starting with a mutant of Saccharomyces cereuisiue lacking glucokinase and both the hexokinase isozymes Pl and P2, strains were constructed, by genetic crosses, that carry single glucose-phosphorylating enzymes. The Pl and P2 isozymes and a structurally a...

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    ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS 182, 639-645 (1977) Physiological Role of Glucose-Phosphorylating Enzymes in Saccharomyces cerevisiae ZITA LOB0 AND P. K. MAITKA Tata Institute of Fundamental Research, Bombay 400005, India Received January 13, 1977 Starting with a mutant of Saccharomyces cereuisiue lacking glucokinase and both the hexokinase isozymes Pl and P2, strains were constructed, by genetic crosses, that carry single glucose-phosphorylating enzymes. The Pl and P2 isozymes and a structurally altered form of Pl hexokinase were partially purified from these strains. Hexokinases Pl, P2, and the altered Pl enzyme, respectively, phosphorylate fructose nearly four, two, and ten times as fast as they phosphorylate glucose. Strains bearing Pl show a pro- nounced Pasteur reaction and phosphorylate glucose, fructose, and mannose faster than those bearing the P2 isozyme. However, there is no appreciable difference between these two hexokinases in regard to the rate and the extent of growth that they sustain. The ability of yeast to grow on a particular sugar is contingent only upon the presence of an enzyme that phosphorylates it. Glucokinase seems to be responsible for catalyzing nearly half of the glucose flux in the wild type yeast. Strains bearing glucokinase alone do show a Pasteur effect. The yeast Saccharomyces cerevisiae is known to have three enzymes for the phos- phorylation of glucose; two of these are hexokinases Pl and P2 (1) known to act also on mannose and fructose, while the third enzyme glucokinase (2) does not use fructose as a substrate. The role of these enzymes in the physiology of the cell is not clear, except that glucokinase is dispensa- ble in fructose metabolism (2). The multi- plicity of these enzymes in the wild type yeast makes such a study inherently diffi- cult. The availability of strains carrying one of these enzymes to the exclusion of the other two should help us to investigate the role of each of these enzymes in sugar metabolism. In the course of genetic studies of yeast hexokinases, we constructed strains hav- ing only a single enzyme for glucose phos- phorylation. We present here the results of some of our experiments on growth and glycolytic rates of these strains. These re- sults show that the growth of yeast on a sugar is determined merely by the pres- ence of an enzyme that phosphorylates it, irrespective of whether it is hexokinase Pl or P2. MATERIALS AND METHODS Strains. A mutant of S. cerevisiae lacking the hexokinase enzymes, 711, has been described (2). This grew on glucose because it had glucokinase. This strain was mutagenized with N-methyl-W-ni- tro-N-nitrosoguanidine in yeast extract-peptone medium (YEP’) containing 50 mM glucose-free ga- lactose, and a glucokinase-negative mutant was iso- lated by replica plating from a YEP gala&se plate to a YEP glucose plate as a glucose-negative colony. This mutant, called 611, did not have any glucoki- nase or hexokinase activity, did not grow on glucose, fructose or mannose, but grew on YEP galactose medium. The genotype of the mutant, determined by independent genetic analysis, was hxkl hxk2 glkl , where hzkl and hxk2 stand for genes coding for Pl and P2 hexokinases, respectively, and glkl for the genetic determinant for glucokinase synthesis. Strains wild type for these characters used were HSC, carrying mating type a and obtained from Dr. S. N. Kakar, and X2928-3D-1C carrying the markers a ao!el trpl gall leul his2 w-a3 met14 and obtained from the Yeast Genetic Stock Center. Standard methods (3) were used in making crosses, isolating 1 Abbreviations used: YEP, yeast extract-pep- tone; 3P-glyceric acid, 3-phosphoglyceric acid; 39- glycerate, 3-phosphoglycerate; F/G ratio, velocity of phosphorylation of fructose relative to that of glu- cose; DEAE, diethylaminoethyl. 639 Copyright 0 1977 by Academic Press, Inc. All rights of reproduction in any form reserved. ISSN 0003-9661

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