Publications

The pH kinetic behavior of several rat fructose-2,6-bisphosphatase forms was analyzed. The bisphosphatase maximal velocity of the hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase was optimal at pH 5, but decreased to 12% of the optimal value in the pH range 7.0-7.5. This decrease depended on deprotonation of a group with a pK of 5.7. In contrast, the separate bisphosphatase domain, a 30-amino acid COOH-terminal truncated form (CT30) of the liver enzyme, and the skeletal muscle bifunctional enzyme exhibited pH-insensitive maximal velocities which were 5-10-fold higher than that of the bisphosphatase of the liver bifunctional enzyme at pH 7.0-7.5. The pK values of the C-2 and C-6 phosphoryl groups were 6.0 and 5.75, respectively, as determined by 31P NMR. Analysis of log kcat/Km versus pH profiles revealed two pK values, one at 6.1, which probably is a substrate pK, and the other at 8.4, which represents an enzyme group. Protein kinase-catalyzed phosphorylation of the liver isoform activated the bisphosphatase, and the pK of the group seen in the kcat profile was increased from 5.7 to 6.4. However, phosphorylation of the CT30 mutant had no effect on the bisphosphatase. The data indicate that NH2- and COOH-terminal interactions in the liver bifunctional enzyme affect the pH dependence of the fructose-2,6-bisphosphatase and its activation by phosphorylation.

The role of Cys-138 in the catalysis of the skeletal muscle 6-phosphofructo-2-kinase reaction was investigated by mutating this residue to serine, glutamine and alanine, expressing the mutants in E. coli with a T7 RNA polymerase-based expression system, and analyzing their kinetic properties. The Cys138Ala mutant had greatly diminished activity, while the Cys138Ser and Cys138Gln mutants had maximal velocities 2-3 fold higher than the wild-type enzyme. It was concluded that Cys-138 does not act as a base catalyst in the kinase reaction, but that it plays a significant structural role in the enzyme's active site.

The role of the NH2-terminal region of the liver and skeletal muscle 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatases was investigated, as well that of a mutant of the liver isoform lacking the first 22 amino acids, by the overexpression of these enzymes in Escherichia coli and the comparison of their kinetic properties. The muscle isoform and the deletion mutant had Km values for fructose 6-phosphate which were 50- and 20-fold higher, respectively, than that of the liver isoform, and the bisphosphatase maximal velocity of the liver deletion mutant was 4-fold higher than that of the native liver isoform. Phosphorylation of the liver isoform increased bisphosphatase activity by 2-3-fold and the Km for fructose 6-phosphate of the 6-phosphofructo-2-kinase by 10-15-fold, but these kinetic effects were greatly diminished for the deletion mutant despite equivalent phosphorylation by cAMP-dependent protein kinase. Arg-173 of the skeletal muscle isoform was found to be functionally equivalent to the residue corresponding to the essential fructose 6-phosphate binding residue of the liver kinase domain, Arg-195. The results suggest that 1) the NH2-terminal regions of the liver and skeletal muscle isoforms are important determinants of fructose 6-phosphate affinity, and 2) the initial 22 amino acids of the liver isoform exert an inhibitory influence on the bisphosphatase and mediate, at least in part, the response of both activities of the enzyme to cAMP-dependent phosphorylation.

The mechanism by which cAMP-dependent protein kinase-catalyzed phosphorylation modulates the activities of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase was examined after site-specific mutation of the cAMP-dependent phosphorylation site (Ser32) to aspartic acid or alanine. The mutant and wild-type enzymes were overexpressed in Escherichia coli in a rich medium to levels as high as 30 mg/liter and were then purified to homogeneity. The kinetic properties of the Ser32-Ala mutant were identical with the dephosphorylated wild-type bifunctional enzyme. Mutation of Ser32 to aspartic acid mimicked several effects of cAMP-dependent phosphorylation: there was an increase in the Km for fructose 6-phosphate for 6-phosphofructo-2-kinase and an increase in the maximal velocity of fructose-2,6-bisphosphatase. Fructose-2,6-bisphosphatase activity of the Ser32-Asp mutant was 75% that of the phosphorylated wild-type enzyme, the mutant's kinase reaction had an identical dependence on fructose 6-phosphate, while its maximum velocity was only 60% that of the phosphorylated wild-type enzyme over a wide pH range. Furthermore, catalytic subunit-catalyzed in vitro phosphorylation of the Ser32-Ala mutant on Ser33 increased the Km for fructose 6-phosphate by 4-fold for the 6-phosphofructo-2-kinase. The results support the hypothesis that Ser32 is an important residue in the regulation of the activities of the bifunctional enzyme and that phosphorylation of Ser32 can be functionally substituted by aspartic acid. The results suggest a role for negative charge in the effect of phosphorylation.

To identify those residues involved in fructose 6-phosphate binding to the kinase domain of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase site-directed mutations were engineered at Lys194, Arg195, Arg230, and Arg238. The mutant enzymes were purified to homogeneity by anion exchange and Blue-Sepharose chromatography and/or substrate elution from phosphocellulose columns. Circular dichroism experiments demonstrated that all of the single amino acid mutations had no effect on the secondary structure of the protein. In addition, when fructose-2,6-bisphosphatase activity was measured, all mutants had Km values for fructose 2,6-bisphosphate, Ki values for fructose 6-phosphate, and maximal velocities similar to that of the wild-type enzyme. Mutation of Arg195––Ala, or His, had little or no effect on the maximal velocity of the kinase but increased the Km for fructose 6-phosphate greater than 3,000-fold. Furthermore, the Ka for phosphate for Arg195Ala was increased 100-fold compared with the wild-type enzyme. Mutation of Lys194––Ala had no effect on maximal velocity or the Km for fructose 6-phosphate. Mutation of either Arg230 or Arg238––Ala increased the maximal velocity and the Km for fructose-6 phosphate of the kinase by 2-3-fold but had no effect on fructose-2,6-bisphosphatase. However, the Km values for ATP of the Arg230Ala and Arg238Ala mutants were 30-40-fold higher than that for the wild-type enzyme. Mutation of Gly48––Ala resulted in a form with no kinase activity, but fructose-2,6-bisphosphatase activity was identical to that of the wild-type enzyme. The results indicate that: 1) Arg195 is a critical residue for the binding of fructose 6-phosphate to the 6-phospho-fructo-2-kinase domain, and that interaction of the sugar phosphate with Arg195 is highly specific since mutation of the adjacent Lys194––Ala had no effect on fructose 6-phosphate binding; 2) Arg195 also play an important role in the binding of inorganic phosphate; and 3) Gly48 is an important residue in the nucleotide binding fold of 6-phosphofructo-2-kinase and that both Arg230 and Arg238 are also involved in ATP binding; and 4) the bifunctional enzyme has two separate and independent fructose 6-phosphate binding sites.

During refeeding after a brief period of starvation, glucose carbon is deposited into hepatic glycogen by both a direct and an indirect route. In the indirect route glucose is first metabolized to 3-carbon precursors, which then transverse the gluconeogenic pathway before being deposited into glycogen. Recent studies have yielded widely different estimates of the percentage of glucose carbon that follows the indirect route. Work summarized here demonstrates that the relative contributions of glucose carbon to hepatic glycogen formation by the indirect and direct pathways are greatly dependent on experimental design, and at least in vitro, are possibly dependent on the extent of glycogen/glucose 1-P recycling. Under physiological refeeding conditions in vivo, both pathways are used, each contributing approximately 50% of the amount of carbon appearing in glycogen. The level of glucokinase activity does not appear to be responsible for poor glucose utilization in liver. Poor glucose utilization in isolated liver preparations may result from the absence of a neurophysiological feedback loop that senses the arterial/portal glucose gradient and then regulates whole liver glucose uptake.

A minimal model of glycogen metabolism can allow the estimation of the flux rates in the glycogen pathway from the time course of the intermediates in the pathway, measured during substrate administration and hormonal stimulation. The comprehensive model of El-Refai & Bergman (Am. J. Physiol. 231, 1608, 1976) consisting of six compartments and 26 non-estimable parameters has successfully accounted for the responses of hepatic glycogenic intermediates in response to a glucose load in hepatocytes (Katz et al., J. biol. Chem. 253, 4530, 1978), in perfused liver (Nordlie et al., J. biol. Chem. 255, 1834, 1980) and during refeeding in vivo (Van DeWerve & Jeanrenaud, Am. J. Physiol. 247, E271, 1984). The comprehensive model is here reduced to a minimal model, consisting of five compartments representing extracellular and intracellular glucose, glucose-phosphate, uridine diphosphate glucose (UDPG), glycogen, and five parameters estimated from the hepatic response to a given stimulus. Estimation of these parameters requires the measurement of the net hepatic glucose balance, the net gluconeogenic flux, and the time course of glycogenic intermediates responding to a hormone or substrate stimulus. The hepatic glycogenolytic response predicted by the comprehensive model in response to an increase in glucagon is closely fitted by the minimal model. When Gaussian distributed random error was added, 0-5% SD in the glucose and glycogen compartments and 0-10% SD in the glucose-phosphate and UDPG compartments, the hepatic response predicted by the minimal model was virtually free of the added error, and the model parameters were found to be within 30% of their true values. When the minimal model was used to interpret the experimental response to an increase in glucose concentration it predicted that: (1) glucokinase can phosphorylate glucose at rates similar to maximal rates of net glycogen synthesis; (2) futile cycling at the glycogen/glucose-1-phosphate level can limit glycogen synthesis; and (3) glucose-6-phosphatase inhibition by glucose has a significant role in net glycogen synthesis.

The coupling from a laser to a thin-film optical waveguide by a prism coupler composed of a birefringent material can be strongly dependent on the orientation of the optic axis. It is shown that when the effective index of the wave guided by the film lies between the ordinary and extraordinary refractive indices of the prism, the coupling depends strongly on the orientation of the optic axis. Differences in orientation of only a few degrees separate ranges of orientation in which strong coupling occurs from ranges in which there is no coupling. The orientation dependence of the coupling is considered both for the pure mode case wherein the optic axis lies in the plane of incidence and for the case when the optic axis is rotated out of the plane of incidence, so that mode coupling occurs. When the effective refractive index of the guided wave is less than both the ordinary and extraordinary refractive indices of the prism, the coupling properties are found to be similar to those obtained with an isotropic prism.