Steady-state kinetic properties of phosphoglycerate kinase of mung beans.
Steady-state kinetics of the action of mung bean phosphoglycerate kinase have been investigated using 3-phosphoglycerate and ATP as substrates in the presence of Mg2+ ions. Keeping a constant and high Mg2+ concentration and varying the concentration of one of the substrates (ATP or 3-phosphoglycerates) at several fixed concentrations of the other substrate (3-phosphoglycerate or ATP), the Km values of Mg.ATP2- and 3-phosphoglycerate were found to be 0.42 and 0.60 mM, respectively. These values are independent of the concentration of the other substrate. A limiting value of Vmax, where the enzyme is saturated with both the substrates, was found to be 39.4 mumoles product formed per min per mg enzyme protein. This corresponds to a turnover number equal to 31.5 sec-1 (for molecular weight of the enzyme equal to 48,000). If [Mg2+] and [ATP4-] are held equal and varied together at several fixed concentrations of 3-phosphoglycerate, deviations from Michaelis-Menten kinetics (non-linear Lineweaver-Burk plots) are observed at lower values of ATP4- and Mg2+ (less than 0.1 mM), giving rise to apparent sigmoidicity in the rate versus [ATP4-] plots. It has been suggested that the real substrate for this enzyme is the Mg.ATP2- complex (and not free ATP4-). The complex dissociates at lower values of [Mg2+] and [ATP4-]. The resulting disproportionate decrease in the concentration of the complex brings about a steeper fall in the rate of reaction than is required by the Michaelis-Menten equation, giving rise to an apparent sigmoidicity.(ABSTRACT TRUNCATED AT 250 WORDS)
Kumar
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Department of Biochemistry, Banaras Hindu University, Varanasi.
Malhotra
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D000255:Adenosine Triphosphate; D007887:Fabaceae; D005988:Glyceric Acids; D007700:Kinetics; D008274:Magnesium; D010735:Phosphoglycerate Kinase / Q000378:metabolism*; D010944:Plants / Q000201:enzymology*; D010946:Plants, Medicinal
D005988:Glyceric Acids; C005156:3-phosphoglycerate; D000255:Adenosine Triphosphate; D010735:Phosphoglycerate Kinase; D008274:Magnesium