Sulphate-activated phosphorylase b: the pH-dependence of catalytic activity

A new interpretation of sulfate activation of rabbit muscle glycogen phosphorylase

It is widely known that sulfate ion at high concentration serves like an allosteric activator of glycogen phosphorylase (GP). Based on the crystallographic studies on GP, it has been assumed that the sulfate ion is bound close to the phosphorylatable Ser¹⁴ site of nonactivated GP, causing a conformational change to catalytically-active GP. However, there are also reports that sulfate ion inhibits allosterically-activated GP by preventing the phosphate substrate from attaching to the catalytic site. In the present study, using a high concentration of sulfate ion, significant enhancement of GP activity was observed when macromolecular glycogen was used as substrate but not when smaller maltohexaose was used. In glycogen solution, nonreducing-end glucose residues are localized on the surface of glycogen and are not distributed homogenously in the solution. Using cyclodextrin-immobilized column chromatography, we found that sulfate at high concentration promoted GP-dextrin binding through the dextrin-binding site (DBS) located away from the catalytic site. This result is consistent with the properties of the DBSs found in glycogen-debranching enzyme and β-amylase. Therefore, we propose a new interpretation of the sulfate activation of GP, wherein sulfate ions at high concentration promote glycogen-binding to the DBS directly, and glycogen-binding to the catalytic site indirectly. Our findings were successfully applied to the affinity purification of porcine brain GP.

The inhibition of human farnesyl pyrophosphate synthase by nitrogen-containing bisphosphonates. Elucidating the role of active site threonine 201 and tyrosine 204 residues using enzyme mutants

Farnesyl pyrophosphate synthase (FPPS) is the major molecular target of nitrogen-containing bisphosphonates (N-BPs), used clinically as bone resorption inhibitors. We investigated the role of threonine 201 (Thr201) and tyrosine 204 (Tyr204) residues in substrate binding, catalysis and inhibition by N-BPs, employing kinetic and crystallographic studies of mutated FPPS proteins. Mutants of Thr201 illustrated the importance of the methyl group in aiding the formation of the Isopentenyl pyrophosphate (IPP) binding site, while Tyr204 mutations revealed the unknown role of this residue in both catalysis and IPP binding. The interaction between Thr201 and the side chain nitrogen of N-BP was shown to be important for tight binding inhibition by zoledronate (ZOL) and risedronate (RIS), although RIS was also still capable of interacting with the main-chain carbonyl of Lys200. The interaction of RIS with the phenyl ring of Tyr204 proved essential for the maintenance of the isomerized enzyme-inhibitor complex. Studies with conformationally restricted analogues of RIS reaffirmed the importance of Thr201 in the formation of hydrogen bonds with N-BPs. In conclusion we have identified new features of FPPS inhibition by N-BPs and revealed unknown roles of the active site residues in catalysis and substrate binding.

Probing the active site of Corynebacterium callunae starch phosphorylase through the characterization of wild-type and His334-->Gly mutant enzymes

His334 facilitates catalysis by Corynebacterium callunae starch phosphorylase through selective stabilization of the transition state of the reaction, partly derived from a hydrogen bond between its side chain and the C-6 hydroxy group of the glucosyl residue undergoing transfer to and from phosphate. We have substituted His334 by a Gly and measured the disruptive effects of the site-directed replacement on active site function using steady-state kinetics and NMR spectroscopic characterization of the cofactor pyridoxal 5'-phosphate and binding of carbohydrate ligands. Purified H334G showed 0.05% and 1.3% of wild-type catalytic center activity for phosphorolysis of maltopentaose (kcatP = 0.033 s(-1)) and substrate binding affinity in the ternary complex with enzyme bound to phosphate (Km = 280 mm), respectively. The 31P chemical shift of pyridoxal 5'-phosphate in the wild-type was pH-dependent and not perturbed by binding of arsenate. At pH 7.25, it was not sensitive to the replacement His334-->Gly. Analysis of interactions of alpha-d-glucose 1-phosphate and alpha-d-xylose 1-phosphate upon binding to wild-type and H334G phosphorylase, derived from saturation transfer difference NMR experiments, suggested that disruption of enzyme-substrate interactions in H334G was strictly local, affecting the protein environment of sugar carbon 6. pH profiles of the phosphorolysis rate for wild-type and H334G were both bell-shaped, with the broad pH range of optimum activity in the wild-type (pH 6.5-7.5) being narrowed and markedly shifted to lower pH values in the mutant (pH 6.5-7.0). External imidazole partly restored the activity lost in the mutant, without, however, participating as an alternative nucleophile in the reaction. It caused displacement of the entire pH profile of H334G by + 0.5 pH units. A possible role for His334 in the formation of the oxocarbenium ion-like transition state is suggested, where the hydrogen bond between its side chain and the 6-hydroxyl polarizes and positions O-6 such that electron density in the reactive center is enhanced.

The clinical chemistry of inorganic sulfate

Although inorganic sulfate is an essential and ubiquitous anion in human biology, it is infrequently assayed in clinical chemistry today. Serum sulfate is difficult to measure accurately without resorting to physicochemical methods, such as ion chromatography, although many other techniques have been described. It is strongly influenced by a variety of physiological factors, including age, diet, pregnancy, and drug ingestion. Urinary excretion is the principal mechanism of disposal for the excess sulfate produced by sulfur amino acid oxidation, and the kidney is the primary site of regulation. In renal failure, sulfoesters accumulate and hypersulfatemia contributes directly to the unmeasured anion gap characteristic of the condition. In contrast, sulfate in urine is readily assayed by a number of means, particularly nephelometry after precipitation as a barium salt. Sulfate is most commonly assayed today as part of the clinical workup for nephrolithiasis, because sulfate is a major contributor to the ionic strength of urine and alters the equilibrium constants governing saturation and precipitation of calcium salts. Total sulfate deficiency has hitherto not been described, although genetic defects in sulfate transporters have been associated recently with congenital osteochondrodystrophies that may be lethal. New insights into sulfate transport and its hormonal regulation may lead to new clinical applications of sulfate analysis in the future.

Phosphonate and alpha-fluorophosphonate analogue probes of the ionization state of pyridoxal 5'-phosphate (PLP) in glycogen phosphorylase

To investigate the role of the essential cofactor pyridoxal phosphate in rabbit muscle glycogen phosphorylase catalysis, two phosphonate analogues of pyridoxal phosphate, 5'-deoxypyridoxal 5'-methylenephosphonic acid and 5'-deoxypyridoxal 5'-difluoromethylenephosphonic acid, have been prepared and reconstituted into apophosphorylase b. UV/Vis spectroscopic and 31P and 19F NMR studies confirmed the successful reconstitution and revealed significant changes in phosphate environment upon nucleotide activation. Both such reconstituted enzymes had activities of approximately 25%-30% of that observed in the native enzyme, while K(m) values were similar to those of the native enzyme. Very similar dependences upon pH of Vmax, K(m), and Vmax/K(m) were found for the two reconstituted enzyme derivatives and the native enzyme despite the considerable difference in phosphonic acid pKa values. These results suggest that pyridoxal phosphate does not function as an essential acid/base catalyst in glycogen phosphorylase; rather they suggest that the cofactor phosphate moiety remain dianionic throughout catalysis and functions as an essential dianion. Mechanistic implications of these findings are discussed.