Regulation of Glucose 6-Phosphate Dehydrogenase in Blue-Green Algae

Abstract

Glucose 6-phosphate dehydrogenase (EC 1. 1.1. 49) has been partially purified from Anacystis nidulans and Anabaena flos-aquae by means of anmnonium sulfate fractionation and exclusion gel chromatography and the kinetic properties de-termined.Glucose 6-phosphate dehydrogenase from these blue-green algae exhibits Michaelis-Menten kinetics at pH 6.7. At this pH, Km values of 0.37 mM for glucose 6-phosphate and 10 zM for NADP were determined. At a pH above 7.4, the enzyme exhibits sigmoidal kinetics with respect to glucose 6-phosphate saturation but the saturation curve for NADP remains hyperbolic.ATP is an inhibitor of the enzyme competitively with NADP with a Ki of 2 to 5 mM. NADPH inhibits the enzyme competitively with glucose 6-phosphate. The inhibition curves for NADPH are hyperbolic at pH 6.7 and sigmoidal at pH 8.6.The significance of these in ritro kinetics are discussed relative to in vivo data on the control of glucose 6-phosphate turnover in blue-green algae.The blue-green algae are procaryotic organisms with a generally obligate photoautotrophic mode of growth (22). Their mechanism for fixing CO, seems to be similar to the mechanism exhibited by higher plants, but To whom correspondence should be addressed.and Bassham (17). The relationship between the degradative shunt and photosynthesis has come under scrutiny only recently. It has been suggested that the pentose phosphate pathway may be regulated by photosynthesis, and that this regulation may be expressed at the first enzyme of the pathway, glucose-6-P dehydrogenase (16).Pelroy and Bassham (17) have found that photosynthesis causes an increase in the glucose-6-P pool size. In view of the finding that the pool size of 6-P-gluconate is very small during photosynthesis, it has been suggested that the blockage occurs at the point of conversion of glucose-6-P to 6-P-gluconate (16,17). Such data indicate that photosynthesis may regulate the flow of carbon through the pentose phosphate pathway by controlling the activity of glucose-6-P dehydrogenase. A variety of factors may contribute to the apparent light-promoted decrease in dehydrogenase activity. The enzyme isolated from the sweet potato tuber displays decreased activity in the presence of either NADPH or ATP (11). During photosynthesis, noncyclic electron transport results in an increased cellular NADPH concentration, whereas cyclic electron flow causes an increase in the cellular ATP concentration. Such increases may reduce the rate of carbon flow through the pentose phosphate pathway via suppression of glucose-6-P dehydrogenase activity. It has recently been reported that ribulose-1, 5-diP, a Calvin cycle intermediate which also shows a marked increase during photosynthesis, causes diminished dehydrogenase activity (16). In addition to the possible enzyme regulation via Calvin cycle intermediates and electron flow products, light-induced pH changes in the extrathylakoidal environment may cause an alteration in enzyme activity. Neumann and Jagendorf (13) have demonstrated that light causes ...

Uptake and Utilization of Sugar Phosphates by Anabaena flos-aquae

“…A block in glucose-6-P metabolism in the light has been demonstrated and the kinetics of this blockage investigated (6). Additional evidence that refixation of CO2 is not a major factor is provided by light saturation experiments (data not shown) in which the inhibition of glucose-6-P turnover is maximal at a light intensity much lower than that which supports appreciable CO2 fixation.…”

“…Our CO2 fixation studies suggested the efficacy of this approach. The stimulation of CO2 fixation in Anabaena by glucose-6-P is promising since this compound has been implicated as an important branch point intermediate in blue-green algae, with its turnover being controlled by some product(s) of photosynthesis (6,16). Our goal, therefore, is to demonstrate that glucose-6-P enters Anabaena cells intact, turns over in a manner consistent with previous studies on in vivo pool size changes (8,13), and affects several physiological functions.…”

“…Finally, there is the question of what regulates glucose-6-P turnover. There are several reports (3,6,8,16) which suggest that either ribulose-1,5-diP, ATP, or NADPH, all of which increase in the light, can prevent turnover of glucose-6-P by feedback control to glucose-6-P dehydrogenase. In order to probe this question, glucose-6-P turnover was measured in the presence or absence of the electron transport inhibitor DCMU and the uncoupler FCCP4 (Table I).…”

“…Grossman and McGowan (6) have investigated the kinetics of glucose-6-P dehydrogenase in Anacystis and Anabaena and have determined regulation of the enzyme's catalytic activity by pH, NADPH, and ATP. These data suggest that if glucose-6-P could be assimilated in the dark, its turnover via the pentose phosphate pathway should go unhindered.…”

Light Modulation of Glucose-6-Phosphate Dehydrogenase

“…Rather it seems more likely that glucose-6-P dehydrogenase may be inactivated within the chloroplast by the LEM and inhibited by NADPH. It is interesting to note that the enzyme from Anacystis nidulans is inactivated by light (7) and, like the higher plant chloroplastic enzyme, sensitive to NADPH (8).…”