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PubMed1991

Pyrophosphate-dependent phosphofructokinase, an anaerobic glycolytic enzyme?

Recent evidence indicates that in as diverse organisms as unicellular eukaryotes, higher plants and prokaryotes, anaerobic glycolysis relies on a pyrophosphate-dependent phosphofructokinase instead of the classical ATP-dependent enzyme. This difference in phosphoryl donor specificity does not necessarily reflect a primitive metabolism, as thought earlier, but could rather be the result of convergent evolution, fostered by the energetic advantage conferred to the cell when glycolysis is the sole source of ATP.

Adenosine Triphosphate↗
PubMed1991

Substrate antagonism in the kinetic mechanism of E. coli phosphofructokinase-1.

In the presence of its allosteric activator GDP, the major phosphofructokinase-1 from Escherichia coli K12 follows Michaelis-Menten kinetics. The kinetic behavior observed at steady-state using different concentrations of the substrates ATP and fructose-6-phosphate and the pattern of inhibition by the substrate analogs adenylyl-(beta, gamma-methylene)-diphosphonate and D-arabinose-5-phosphate are consistent with a random sequential mechanism in rapid equilibrium, rather than with an ordered binding as was suggested earlier. However, ATP and fructose-6-phosphate do not bind independently to the same active site, since the apparent affinity for one substrate is decreased about 20-fold when the other substrate is already bound. The antagonism between ATP and fructose-6-phosphate shows that a negative interaction occurs during the reaction with E. coli phosphofructokinase-1 which must be considered in addition to its allosteric properties.

Adenosine Triphosphate↗
PubMed1994

Heterologous expression of an active rat regulatory protein of glucokinase.

The cDNA presumed to encode the rat liver regulatory protein of glucokinase has been expressed in Escherichia coli and a partially soluble protein has been obtained. This recombinant protein was partially purified and found to have the same apparent molecular mass as the regulatory protein purified from rat liver. Like the latter, it inhibited rat liver glucokinase competitively with respect to glucose and its effect was sensitive to fructose 6-phosphate and fructose 1-phosphate.

Animals↗
PubMed1996

Effects of temperature shifts on the activities of Neurospora crassa glycogen synthase, glycogen phosphorylase and trehalose-6-phosphate synthase.

Conidiospore germlings of Neurospora crassa submitted to a heat shock at 45 degrees C accumulate trehalose and degrade glycogen. The opposite occurs upon reincubation at a physiologic temperature (30 degrees C). These observations suggest a temperature-dependent mechanism for the preferential synthesis of one or the other sugar reserve. Here we show that concomitant with these shifts of temperature, occurred reversible changes in the activities of glycogen synthase and phosphorylase. Glycogen synthase was inactivated at 45 degrees C while phosphorylase was activated. The reverse was true when the cells were shifted back to 30 degrees C. Addition of cycloheximide did not prevent the reversible enzymatic changes, which remained stable after gel filtration. Apparently, the effects of temperature shifts occurred at the level of reversible covalent enzymatic modifications. Trehalose-6-phosphate synthase properties were also affected by temperature. For instance, the enzyme was less sensitive to in vitro inhibition by inorganic phosphate at 50 degrees C than at 30 degrees C. Fructose-6-phosphate partially relieved the inhibitory effect of phosphate at 30 degrees C but not at 50 degrees C. These effects of the assay temperature, inorganic phosphate, and fructose-6-phosphate, on trehalose-6-phosphate synthase activity, were more evident for crude extracts obtained from heat-shocked cells. Altogether, these results may contribute to explain the preferential accumulation of trehalose 45 degrees C, or that of glycogen at 30 degrees C.

Cycloheximide↗
PubMed1992

Purification and characterization of phosphofructokinase in bovine parotid gland.

1. Phosphofructokinase (PFK) was purified from bovine parotid gland to 750-fold with the specific activity of 67.5 units/mg protein by Cibacron Blue F3GA affinity chromatography, and TSK DEAE-5PW ion-exchange and TSK G4000SW size exclusion chromatographies on HPLC. 2. On gel-filtration, molecular weight of the native PFK was estimated to 400,000. 3. PFK was a heterotetramer composed of three kinds of subunit with molecular weights of 92,000 (C-type), 88,000 (M-type) and 86,000 (L-type), by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Densitometrically, relative amounts of C-, M- and L-type subunit were 1:1:2. 4. Under the physiological conditions of fructose 6-phosphate (Fru-6-P) and ATP concentrations and pH, PFK activity was suppressed and hardly detectable. 5. Fru-6-P relieved PFK from the ATP inhibition. 6. Fructose 2,6-bisphosphate (Fru-2,6-P2) and AMP activated PFK with a reduction of S0.5 for Fru-6-P and subunit cooperativity. Fru-2,6-P2 was more effective than AMP.

Adenosine Monophosphate↗
PubMed1993

Enzyme-to-enzyme tunnelling between phosphoglucoisomerase and phosphofructokinase.

1. Cross-linked and permeabilized rat erythrocytes were incubated for 2-5 min at 37 degrees C in the presence of ATP and either D-[U-14C]glucose 6-phosphate (3 mM) mixed with unlabelled D-fructose 6-phosphate (1 mM) or D-[U-14C]fructose 6-phosphate (1 mM) mixed with unlabelled D-glucose 6-phosphate (3 mM). 2. The contribution of molecules derived from the radioactive ketohexose ester relative to the total amount of newly formed D-fructose 1,6-bisphosphate was lower than the time-related average value for such a relative contribution in the pool of D-fructose 6-phosphate. 3. From such a difference, it was calculated that, under the present experimental conditions, 13.1 +/- 2.0% of the molecules of D-fructose 1,6-bisphosphate formed during incubation are directly derived from D-glucose 6-phosphate by a process of enzyme-to-enzyme channelling between phosphoglucoisomerase and phosphofructokinase, rather than originating from the free pool of D-fructose 6-phosphate. 4. A comparable value of 13.2 +/- 3.2% was reached when the process of enzyme-to-enzyme tunnelling was judged from the 3H/14C ratio in D-fructose 1,6-bisphosphate formed by permeabilized erythrocytes exposed for 5-15 min to D-glucose 6-phosphate (3 or 5 mM) mixed with tracer amounts of both D-[1-14C]glucose 6-phosphate and D-[2-3H]glucose 6-phosphate.

Animals↗
PubMed1992

Structure of a ternary complex of an allosteric lactate dehydrogenase from Bacillus stearothermophilus at 2.5 A resolution.

We report the refined structure of a ternary complex of an allosterically activated lactate dehydrogenase, including the important active site loop. Eightfold non-crystallographic symmetry averaging was utilized to improve the density maps. Interactions between the protein and bound coenzyme and oxamate are described in relation to other studies using site-specific mutagenesis. Fructose 1,6-bisphosphate (FruP2) is bound to the enzyme across one of the 2-fold axes of the tetramer, with the two phosphate moieties interacting with two anion binding sites, one on each of two subunits, across this interface. However, because FruP2 binds at this special site, yet does not possess an internal 2-fold symmetry axis, the ligand is statistically disordered and binds to each site in two different orientations. Binding of FruP2 to the tetramer is signalled to the active site principally through two interactions with His188 and Arg173. His188 is connected to His195 (which binds the carbonyl group of the substrate) and Arg173 is connected to Arg171 (the residue that binds the carboxylate group of the substrate).

Allosteric Regulation↗