Identification of Asp 549 as the catalytic nucleophile of glycogen-debranching enzyme via trapping of the glycosyl-enzyme intermediate.

Glycogen-debranching enzyme catalyzes the removal of branching from glycogen via a two-step process involving first the transfer of a maltotriosyl unit from the branch to the main chain and second the hydrolysis of the residual alpha-(1,6)-linked glucose moiety. Since the transfer occurs with retention of anomeric configuration, a mechanism involving a maltotriosyl-enzyme species is presumed. 4-Deoxy-alpha-maltotriosyl fluoride functions as an incompetent substrate for this transferase activity since a glycosyl-enzyme species in formed, as witnessed by a "burst" of fluoride release, but turned over only very slowly unless a suitable acceptor such as maltotriose is added, at which point 4-deoxymaltohexaose is released. Peptic proteolysis of this trapped enzyme generated a mixture of peptides which was separated by reverse phase high-performance liquid chromatography, and the glycosylated peptide was located by use of tandem mass spectrometry in the neutral loss mode. Subsequent tandem mass spectrometric experiments on this peptide identified it as one surrounding Asp 549. This amino acid is completely conserved in all alpha-glucanotransferases and alpha-glucosidases belonging to this sequence -related family and is hereby identified as the catalytic nucleophile.

Effects of oligosaccharide binding on glycogen debranching enzyme activity and conformation.

Glycogen debranching enzyme contains two catalytic activities (4-alpha-glucanotransferase and amylo-1,6-glucosidase) on its single polypeptide chain, and they are affected differently by the binding of oligosaccharides. Glucose, maltose, and maltotriose are competitive inhibitors of the amylo-1,6-glucosidase activity measured by the hydrolysis of alpha-glucosyl fluoride, whereas saccharides with four or more glucose units are activators of the same activity, showing apparent "uncompetitive" kinetics. This suggests that they do not bind until the alpha-glucosyl fluoride is bound. In either case the potency of the effect increases with the length of the oligosaccharide chain. On the other hand, all oligosaccharides tested (maltose to maltohexaose, alpha-cyclodextrin, and beta-cyclodextrin) are competitive inhibitors of the transferase activity and also cause a decrease in the intrinsic fluorescence, both functions again increased by chain length, thus indicating that these saccharides do bind to the free enzyme. These interesting results can be reconciled if the extended main chain resulting from the transferase reaction has to be reoriented into a different binding mode in order to position the alpha-1,6-linked side-chain glucose into the correct position for the glucosidase reaction. Therefore, activating oligosaccharides behave kinetically as if they had not been previously bound. It is concluded that the main chain of the natural limit dextrin substrate has a different mode of binding for the two catalytic reactions in order to position properly first the maltotetraosyl side chain in the transferase catalytic site and then the glucosyl side chain in the glucosidase catalytic site.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning, sequencing, and analysis of the cDNA for rabbit muscle glycogen debranching enzyme.

Six peptides were isolated from glycogen debranching enzyme purified from rabbit muscle, and their sequences were determined. A cDNA library made from rabbit muscle using random hexamer primers was screened with oligonucleotide probes constructed in accordance with these peptide sequences. Seven cDNA clones comprising the open reading frame were found, whereas oligo(dT) cDNA libraries yielded no positive clones because of the long 3'-nontranslated region of 2.3 kb. The open reading frame of 4665 bases codes for a 1555-amino-acid protein of M(r) 177,542. Compared to the sequence from human muscle, there are an additional 40 amino acid residues upstream from the N-terminus, and the next 10 residues show no homology. For the remaining 1505 residues, the two sequences exhibit an identity of 93%. The four consensus sequences commonly found at the carboxy termini of beta-strands in the alpha/beta barrel domains of amylases and glucanotransferases are also found in the N-terminal half of the debranching enzyme, suggesting that this structural domain may be present. This and other evidence suggests that the N-terminal half may encompass the transferase activity, leaving the glucosidase activity for the C-terminal half. The latter shows no significant homology to known proteins. An unusual feature of the sequence is the presence of three pairs of adjacent cysteines, which may explain inhibition of the enzyme by organic arsenites.

Multiple phosphate positions in the catalytic site of glycogen phosphorylase: structure of the pyridoxal-5'-pyrophosphate coenzyme-substrate analog.

The three-dimensional structure of an R-state conformer of glycogen phosphorylase containing the coenzyme-substrate analog pyridoxal-5'-diphosphate at the catalytic site (PLPP-GPb) has been refined by X-ray crystallography to a resolution of 2.87 A. The molecule comprises four subunits of phosphorylase related by approximate 222 symmetry. Whereas the quaternary structure of R-state PLPP-GPb is similar to that of phosphorylase crystallized in the presence of ammonium sulfate (Barford, D. & Johnson, L.N., 1989, Nature 340, 609-616), the tertiary structures differ in that the two domains of the PLPP-GPb subunits are rotated apart by 5 degrees relative to the T-state conformation. Global differences among the four subunits suggest that the major domains of the phosphorylase subunit are connected by a flexible hinge. The two different positions observed for the terminal phosphate of the PLPP are interpreted as distinct phosphate subsites that may be occupied at different points along the reaction pathway. The structural basis for the unique ability of R-state dimers to form tetramers results from the orientation of subunits with respect to the dyad axis of the dimer. Residues in opposing dimers are in proper registration to form tetramers only in the R-state.

Structural basis for the activation of glycogen phosphorylase b by adenosine monophosphate.

The three-dimensional structure of the activated state of glycogen phosphorylase (GP) as induced by adenosine monophosphate (AMP) has been determined from crystals of pyridoxalpyrophosphoryl-GP. The same quaternary changes relative to the inactive conformation as those induced by phosphorylation are induced by AMP, although the two regulatory signals function through different local structural mechanisms. Moreover, previous descriptions of the phosphorylase active state have been extended by demonstrating that, on activation, the amino- and carboxyl-terminal domains of GP rotate apart by 5 degrees, thereby increasing access of substrates to the catalytic site. The structure also reveals previously unobserved interactions with the nucleotide that accounts for the specificity of the nucleotide binding site for AMP in preference to inosine monophosphate.

Reassessment of the catalytic mechanism of glycogen debranching enzyme.

The amylo-1,6-glucosidase catalytic activity of glycogen debranching enzyme allows it to hydrolyze alpha-D-glucosyl fluoride, in the absence or presence of glycogen or oligosaccharides, releasing equal amounts of fluoride and glucose at rates comparable to those seen with the natural substrates. 2-Deoxy-2-fluoro-alpha-D-glucosyl fluoride is found to be a poor substrate, rather than the covalent inhibitor that would be expected for a glucosidase which catalyzes hydrolysis of the glycosidic linkage with retention of anomeric configuration. In fact, analysis of the glucosidase reaction by NMR reveals that the debranching enzyme hydrolyzes the glycosidic linkage with inversion of configuration, releasing beta-D-glucose from both alpha-glucosyl fluoride and its natural substrate, the phosphorylase limit dextrin. In contrast, its transferase activity necessarily proceeds with retention of configuration. As has been seen with other "inverting" glycosidases, the debranching enzyme releases beta-D-glucose from beta-D-glucosyl fluoride in the presence of oligosaccharides such as maltohexaose and cyclomaltoheptaose but, unlike the others, not in their absence. An intermediate glucosyl-alpha-(1,6)-cyclomaltoheptaose has been detected by NMR analysis. In the presence of a water-soluble carbodiimide, a single mole of glycine ethyl ester is incorporated into each mole of the debranching enzyme, resulting in its inactivation when measured by the combined assay for both transferase and glucosidase activities. Measurement of the latter two activities independently indicates that it is the transferase activity which is inactivated, while the glucosidase activity, measured with alpha-D-glucosyl fluoride as substrate, is unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

[Injuries due to lap seat belts]. / Hofteselelaesion.

A case of lap seat belt injury with damage to the small intestine is presented. The lap seat belt induces a sense of false security. This type of seat belt should be forbidden and should be replaced by the better three-point-restraint. Patients with seat belt lesions should be admitted to hospital.

Electron microscopy of glycogen degrading enzymes.

Single molecules of glycogen phosphorylase b exhibit images in the electron microscope which are similar in shape and dimension to those derived from X-ray crystallography. Phosphorylase alpha exhibits tetramers but shows dimers in the presence of glucose. Glycogen debranching enzyme appears as a monomer with an unusual crescent or shrimp-like shape, with occasional isologous aggregation to circular dimers. The longest dimension of the monomer is very similar to that of the phosphorylase dimer, 11.5 nm. Strong binding of the debranching enzyme to glycogen is readily visualized in the electron microscope. It is suggested that the distinctive shape of the debranching enzyme may be related to its catalytic function.

Structural changes in glycogen phosphorylase induced by phosphorylation.

A comparison of the refined crystal structures of dimeric glycogen phosphorylase b and a reveals structural changes that represent the first step in the activation of the enzyme. On phosphorylation of serine-14, the N-terminus of each subunit assumes an ordered helical conformation and binds to the surface of the dimer. The consequent structural changes at the N- and C-terminal regions lead to strengthened interactions between subunits and alter the binding sites for allosteric effectors and substrates.

Binding of glycogen, oligosaccharides, and glucose to glycogen debranching enzyme.

The binding of glucose and a series of oligosaccharides to glycogen debranching enzyme was determined by the ability of the saccharides to decrease the rate of reaction of sulfhydryl groups with 5,5'-dithiobis(2-nitrobenzoate) (DTNB). At pH 7.2, the strength of binding increases with chain length from glucose to maltotriose to maltopentaose but not to maltohexaose, and the free energies for binding of the oligosaccharides suggest subsites of equivalent affinities for the four glucose units following the initial reducing moiety. The rate of reaction of DTNB with enzyme saturated with saccharide is the same for all compounds, suggesting that all the saccharides, including glucose, induce the same conformational state. The site of binding may be that which binds the alpha-1,6-linked side chain of the natural limit dextrin substrate. At pH 8.0, this site exhibits similar characteristics, but an additional site, which may bind the four terminal glucose units of the main chain of the natural substrate, is manifested and exhibits different characteristics, including a very low affinity for glucose itself. The binding of glycogen to the debranching enzyme was monitored by centrifugal separation from the protein and exhibits a much lower dissociation constant than that for the oligomers, suggesting that branched polymers have more than one set of subsites.

Distinction between substrate- and enzyme-directed effects of modifiers of rabbit liver phosphorylase a phosphatases.

The natural substrate (phosphorylase a) and two alternative ones (phosphorylated histone and a tetradecapeptide consisting of residues 5-18 of rabbit skeletal muscle phosphorylase a) were used to distinguish the modes of action of some physiologically important effectors of four different molecular forms of rabbit liver phosphorlase a phosphatases. In general, glucose, caffeine, AMP, ADP, Pi, and glucose-1-P showed substrate-directed effects for the holophosphatase forms, since they usually did not affect the activity on histone phosphate and, with one slight exception (Pi), never affected the activity on the tetradecapeptide phosphate. ADP, Pi, and glucose-1-P did affect directly the relative mass (Mr) 35,000 phosphatase, in addition to an inhibition mediated via phosphorylase a. ATP exerted both substrate- and enzyme-directed effects for the Mr 35,000 phosphatase and phosphatases 1 and 2A2, but only a substrate-directed effect for phosphatase 2A1, suggesting that the gamma-subunit of the type 2 phosphatases may prevent ATP binding to the phosphatase. Mg2+ showed substrate-directed effects for phosphatases 1, 2A1, and 2A2, and an additional enzyme-directed effect for the Mr 35,000 phosphatase form. Furthermore, Mg2+ could not abolish ATP inhibition of the tetradecapeptide phosphatase activity, but significantly overcame ATP inhibition of the phosphorylase a phosphatase activity, thus suggesting that its ability to reverse the ATP effect is by a substrate-directed mechanism. The substrate-directed effects seen for the different ligands on the different phosphatase forms strongly indicate the significance of this form of control in the regulation of phosphorylase a phosphatase activities and may serve to narrow the otherwise broad substrate specificities of the major phosphorylase a phosphatase activities in mammalian tissues: phosphatases 1 and 2A.

Pain relief after major abdominal surgery: a double-blind controlled comparison of sublingual buprenorphine, intramuscular buprenorphine, and intramuscular meperidine.

In a double-blind randomized study of three groups of 18 patients scheduled for major abdominal surgery the efficacy and side effects of sublingual buprenorphine were tested and compared to intramuscular meperidine and buprenorphine. Single doses of either 75 mg of meperidine, 0.4 mg of sublingual buprenorphine, or 0.3 mg of intramuscular buprenorphine were used. Patients given buprenorphine as sublingual tablets were significantly more conscious in the immediate postoperative period (Glasgow Coma Scale) than when given buprenorphine or meperidine intramuscularly. Median pain intensity differences (PID) showed equal pain relief, whereas the summarized pain intensity differences (SPID) were significantly higher in the intramuscular buprenorphine group compared to the meperidine group. Three cases of respiratory acidosis in the meperidine group required IPPV treatment, and one case in the intramuscular buprenorphine group required treatment. Sedation and nausea were the most common side effects in all three groups. We conclude that sublingual buprenorphine is useful for relief of postoperative pain and exhibited administrative advantages, when the patients were able to cooperate.

Phosphorylation state of acetyl-coenzyme A carboxylase. I. Linear inverse relationship to activity ratios at different citrate concentrations.

Acetyl-CoA carboxylase and its associated kinase have been purified to homogeneity from rat liver and, together with the catalytic subunit of liver protein phosphatase, used to study the effect of phosphorylation on the carboxylase activity. Phosphatase increases the carboxylase activity, whereas the kinase decreases it. A linear inverse relationship (correlation coefficient = 0.98) exists between phosphate incorporated by the kinase and the specific activity. The kinetics of activation by citrate show an increased Ka and a decreased Vmax for carboxylase preparations with increasing levels of phosphate. On this basis an enzymic test was devised for phosphate incorporated by the kinase. Thus the ratio of activities at 0 and 2 mM citrate is inversely proportional to the phosphate incorporated (correlation coefficient = -0.95), with 0.8 mol of P incorporated per mol of subunit decreasing the activity ratio from 0.5 to 0. This activity ratio method has an inherent internal control which makes it suitable for determining the level of protein-bound phosphate affecting the carboxylase activity in crude tissue extracts, and hence it should be useful for physiological studies. Tryptic maps of carboxylase labeled with radioactive phosphate by the carboxylase kinase indicate that the slightly less than 1 mol of P/mol of subunit is distributed equally between two peptides, whereas cAMP-dependent protein kinase phosphorylates these two sites and a third which may not affect activity.

Phosphorylation state of acetyl-coenzyme A carboxylase. II. Variation with nutritional condition.

Acetyl-CoA carboxylase from liver exhibits a linear inverse relationship between the ratio of enzymic activities at 0 and 2 mM citrate and the extent of phosphorylation by its kinase, and this citrate activity ratio method was used to examine the effect of nutritional conditions on the phosphorylation state of the enzyme. This method showed that the calculated phosphorylation state, being the extent of phosphorylation at sites accessible to carboxylase kinase, was highest in the livers of starved rats, lower in those fed normally, and lower still in starved rats which had been refed for 48 h on a fat-free diet. The actual values were 0.44, 0.26, and 0 mol of P/subunit, respectively, provided that liver samples were frozen rapidly to liquid nitrogen temperatures and extracted with stopping buffers at temperatures well below freezing. Normal homogenization with stopping buffers (containing inhibitors for protein kinases and phosphatases) resulted in much higher calculated phosphorylation states. The effect of nutritional conditions on the phosphorylation state as estimated reported above was confirmed by purifying the carboxylase from livers of rats, measuring the amount of phosphate which could be incorporated by carboxylase kinase, and comparing this with the phosphorylation state calculated from the citrate activity ratio method or the specific activity. Furthermore, treatment with protein phosphatase of carboxylase from starved rats resulted in the largest increase in specific activity, that from the starved/refed rats in the least. Finally, the effects of hyperglycemia on carboxylase and phosphorylase characteristics in the livers of intact rats were ascertained by taking liver samples and preparing crude extracts by the rapid freezing method described above. Hyperglycemia caused a rapid increase in the activity of the carboxylase and a rapid decrease in its putative phosphorylation state as measured by the citrate activity ratio method. Phosphorylase was also dephosphorylated, as indicated by a decrease in phosphorylase a activity. We conclude that the citrate activity ratio method is a valid test for the phosphorylation state of acetyl-CoA carboxylase in crude extracts of tissue.

Gastric volume and pH in children for emergency surgery.

The volume and pH of gastric contents aspirated prior to anaesthesia were measured in 101 children admitted for emergency surgery. The children were aged between 3 months and 15 years. If we define potential patients at risk by means of the volume and pH of the gastric contents, then 50.0% of the children were at risk of aspiration into the lungs. The number of patients at risk was higher in children aged between 6 and 10 years. There was almost the same risk in the groups with abdominal-, urogenital-, and orthopaedic diseases, while the number of patients at risk was less in the group with superficial lesions. The length of fasting time in the child considerably influenced the volume of gastric contents in emergency surgical cases. It is concluded that in children admitted for emergency surgery there is a risk of aspiration of gastric contents into the lungs. The risk is reduced by preanaesthetic fasting. All children admitted for emergency surgery must be carefully evaluated prior to anaesthesia with special reference to gastric aspiration.

31P NMR relaxation studies of the activation of the coenzyme phosphate of glycogen phosphorylase. The role of motion of the bound phosphate.

Spin-lattice and spin-spin relaxation rates (1/T1 and 1/T2) have been determined for the catalytically essential coenzyme phosphate at the active site of glycogen phosphorylase in both activated (R state) and inactive (T state) conformations of the enzyme. Dipolar contributions to 31P relaxation due to exchangeable protons on the phosphate group have been determined by measurement of relaxation rates at different concentrations of H2O and D2O, and field dependence studies have been performed to estimate the contribution of chemical shift anisotropy to the remaining 31P relaxation in D2O. At 109 MHz, dipolar relaxation from exchangeable protons was found to account for 50% of the spin-lattice relaxation for activated phosphorylase in 75% H2O, the remainder being due to chemical shift anisotropy. The spin-lattice relaxation rates in D2O for R-state glycogen phosphorylase are very similar to those measured for other proteins of very different size such as actin (Brauer, M., and B. D. Sykes, 1981, Biochemistry. 20:6767-6775), alkaline phosphatase (Coleman, J. E., I. D. Armitage, J. F. Chlebowski, J. D. Otvos, and A. J. M. S. Uiterkamp, 1979), and phosphoglucomutase (Rhyu, G. I., W. J. Ray, Jr., and J. L. Markley, 1984, Biochemistry. 23:252-260). In inactive (T state) phosphorylase the spin-lattice relaxation rates were almost an order of magnitude slower, while the spin-spin relaxation rates were essentially identical. These results have been analyzed by calculating the theoretically expected 31P relaxation rates in the presence of internal motions that are included in the relaxation calculation using the model-free approach of Lipari and Szabo (1982, J. Am. Chem. Soc. 104:4564-4559). The analysis suggests the coenzyme phosphate is relatively immobilized in the activated enzymic conformation, but in the inactive (Tstate) conformation it is considerably more mobile with a rotational correlation time one to two orders of magnitude smaller. Since the spin-lattice relaxation rate for the active R-state (immobilized) phosphate is similar to that observed in other phosphoenzymes of different size it is suggested that a librational motion on the nanosecond time scale may constitute a common spin-lattice relaxation pathway for phosphates in macromolecules. The consequences of phosphate motion in terms of recent suggestions concerning the environment and the catalytic role of the coenzyme phosphate are discussed.

Rabbit liver phosphorylase a phosphatase: regulation by glucose and caffeine.

The modulation of three molecular forms of liver phosphorylase a phosphatases, the "catalytic" subunit phosphatase (35 000 relative mass (Mr) and phosphatases 2A1 and 2A2 by glucose and caffeine and some physiologically important compounds were studied, using 32P-labelled phosphorylase a obtained from rabbit liver as substrate. Glucose and caffeine showed independent and additive activations. The caffeine effect was seen at micromolar to millimolar concentrations and glucose caused activation even at concentrations below the normal blood glucose level. The nucleotides ATP and AMP, at their presumed physiological concentrations in the liver, were strongly inhibitory. Inhibition by these nucleotides and other inhibitors tested showed varied responses to the presence of the activators glucose and caffeine, depending on the phosphatase form. Thus, significant relief of ATP inhibition was afforded by glucose and caffeine acting independently for the 35 000 Mr phosphatase, whereas relief of inhibition for phosphatases 2A1 and 2A2 required a combination of glucose, caffeine, and Mg2+. The Km of the liver 35 000 Mr phosphatase was about 50 microM for the liver substrate as compared with 4 microM for the muscle substrate. The Km of phosphatase 2A2 was about 16 microM and for phosphatase 2A1 it was about 20 microM, using liver substrate in the absence of any stimulators. Mg2+ inhibited the 35 000 Mr phosphatase, but became stimulatory for phosphatase 2A2 and was an almost obligatory requirement for phosphatase 2A1.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization of glycogen debranching enzyme.

Crystals of glycogen debranching enzyme from rabbit skeletal muscle have been obtained from solutions of polyethylene glycol 8000 (pH 7.3) containing 10 mM-linear oligosaccharides of lengths from three to seven glucose units in alpha-1,4 linkage. Preliminary X-ray precession photographs indicate an orthorhombie unit cell with dimensions of a = 106.4 A, b = 195.7 A and c = 93.0 A. The space group is P212121 with one monomer per asymmetric unit.