Granule-bound starch synthase I in isolated starch granules elongates malto-oligosaccharides processively.
(9/1049)
Isoforms of starch synthase belonging to the granule-bound starch synthase I (GBSSI) class synthesize the amylose component of starch in plants. Other granule-bound isoforms of starch synthase, such as starch synthase II (SSII), are unable to synthesize amylose. The kinetic properties of GBSSI and SSII that are responsible for these functional differences have been investigated using starch granules from embryos of wild-type peas and rug5 and lam mutant peas, which contain, respectively, both GBSSI and SSII, GBSSI but not SSII and SSII but not GBSSI. We show that GBSSI in isolated granules elongates malto-oligosaccharides processively, adding more than one glucose molecule for each enzyme-glucan encounter. Granule-bound SSII can elongate malto-oligosaccharides, but has a lower affinity for these than GBSSI and does not elongate processively. As a result of these properties GBSSI synthesizes longer malto-oligosaccharides than SSII. The significance of these results with respect to the roles of GBSSI and SSII in vivo is discussed. (+info)
Stable, inducible thermoacidophilic alpha-amylase from Bacillus acidocaldarius.
(10/1049)
Bacillus acidocaldarius Agnano 101 produces an inducible thermoacidophilic alpha-amylase. The enzyme production occurs during the stationary phase of growth in the presence of compounds with alpha-1,4-glucosidic linkages. The enzymatic activity is both present in the culture medium and associated with the cells; the enzymes purified from both sources show identical molecular and catalytic properties. The purified amylase has a single polypeptide chain of molecular weight 68,000 and behaves like an alpha-amylase with affinity constants for starch and related substances of 0.8 to 0.9 mg/ml. The pH and temperature optima for activity are 3.5 and 75degreesC, respectively. The amylase is stable at acidic pH (below 4.5). Its thermal stability is strictly dependent upon protein concentration; the half-life at 60degreesC of the amylase in a 70-mug/ml solution is about 5 days. (+info)
Maltose metabolism in the hyperthermophilic archaeon Thermococcus litoralis: purification and characterization of key enzymes.
(11/1049)
Maltose metabolism was investigated in the hyperthermophilic archaeon Thermococcus litoralis. Maltose was degraded by the concerted action of 4-alpha-glucanotransferase and maltodextrin phosphorylase (MalP). The first enzyme produced glucose and a series of maltodextrins that could be acted upon by MalP when the chain length of glucose residues was equal or higher than four, to produce glucose-1-phosphate. Phosphoglucomutase activity was also detected in T. litoralis cell extracts. Glucose derived from the action of 4-alpha-glucanotransferase was subsequently metabolized via an Embden-Meyerhof pathway. The closely related organism Pyrococcus furiosus used a different metabolic strategy in which maltose was cleaved primarily by the action of an alpha-glucosidase, a p-nitrophenyl-alpha-D-glucopyranoside (PNPG)-hydrolyzing enzyme, producing glucose from maltose. A PNPG-hydrolyzing activity was also detected in T. litoralis, but maltose was not a substrate for this enzyme. The two key enzymes in the pathway for maltose catabolism in T. litoralis were purified to homogeneity and characterized; they were constitutively synthesized, although phosphorylase expression was twofold induced by maltodextrins or maltose. The gene encoding MalP was obtained by complementation in Escherichia coli and sequenced (calculated molecular mass, 96,622 Da). The enzyme purified from the organism had a specific activity for maltoheptaose, at the temperature for maximal activity (98 degrees C), of 66 U/mg. A Km of 0.46 mM was determined with heptaose as the substrate at 60 degrees C. The deduced amino acid sequence had a high degree of identity with that of the putative enzyme from the hyperthermophilic archaeon Pyrococcus horikoshii OT3 (66%) and with sequences of the enzymes from the hyperthermophilic bacterium Thermotoga maritima (60%) and Mycobacterium tuberculosis (31%) but not with that of the enzyme from E. coli (13%). The consensus binding site for pyridoxal 5'-phosphate is conserved in the T. litoralis enzyme. (+info)
Molecular and biochemical analysis of MalK, the ATP-hydrolyzing subunit of the trehalose/maltose transport system of the hyperthermophilic archaeon Thermococcus litoralis.
(12/1049)
We report the cloning, sequencing, and expression of malK encoding the ATP-hydrolyzing subunit of the maltose/trehalose transport system of the hyperthermophilic archaeon Thermococcus litoralis. According to the deduced amino acid sequence, MalK consists of 372 amino acids with a calculated molecular weight of 41,787. It shows 47% identity with the MalK protein of Escherichia coli and high sequence conservation in important regions. C-terminal His-tagged MalK was purified. The soluble protein appeared monomeric by molecular sieve chromatography and showed ATPase activity. Enzymatic activity was highest at 80 degrees C with a Km of 150 microM and a Vmax of 0.55 micromol of ATP hydrolyzed/min/mg of protein. ADP was not a substrate but a competitive inhibitor (Ki 230 microM). GTP and CTP were also hydrolyzed. ATPase activity was inhibited by N-ethylmaleimide but not by vanadate. The strong homology found between the components of this archaeal transport system and the bacterial systems is evidence for the evolutionary conservation of the ABC transporters in these two phylogenetic branches. (+info)
Escherichia coli maltose-binding protein is uncommonly effective at promoting the solubility of polypeptides to which it is fused.
(13/1049)
Although it is usually possible to achieve a favorable yield of a recombinant protein in Escherichia coli, obtaining the protein in a soluble, biologically active form continues to be a major challenge. Sometimes this problem can be overcome by fusing an aggregation-prone polypeptide to a highly soluble partner. To study this phenomenon in greater detail, we compared the ability of three soluble fusion partners--maltose-binding protein (MBP), glutathione S-transferase (GST), and thioredoxin (TRX)--to inhibit the aggregation of six diverse proteins that normally accumulate in an insoluble form. Remarkably, we found that MBP is a far more effective solubilizing agent than the other two fusion partners. Moreover, we demonstrated that in some cases fusion to MBP can promote the proper folding of the attached protein into its biologically active conformation. Thus, MBP seems to be capable of functioning as a general molecular chaperone in the context of a fusion protein. A model is proposed to explain how MBP promotes the solubility and influences the folding of its fusion partners. (+info)
Phosphorylase recognition and phosphorolysis of its oligosaccharide substrate: answers to a long outstanding question.
(14/1049)
Phosphorylases are key enzymes of carbohydrate metabolism. Structural studies have provided explanations for almost all features of control and substrate recognition of phosphorylase but one question remains unanswered. How does phosphorylase recognize and cleave an oligosaccharide substrate? To answer this question we turned to the Escherichia coli maltodextrin phosphorylase (MalP), a non-regulatory phosphorylase that shares similar kinetic and catalytic properties with the mammalian glycogen phosphorylase. The crystal structures of three MalP-oligosaccharide complexes are reported: the binary complex of MalP with the natural substrate, maltopentaose (G5); the binary complex with the thio-oligosaccharide, 4-S-alpha-D-glucopyranosyl-4-thiomaltotetraose (GSG4), both at 2.9 A resolution; and the 2.1 A resolution ternary complex of MalP with thio-oligosaccharide and phosphate (GSG4-P). The results show a pentasaccharide bound across the catalytic site of MalP with sugars occupying sub-sites -1 to +4. Binding of GSG4 is identical to the natural pentasaccharide, indicating that the inactive thio compound is a close mimic of the natural substrate. The ternary MalP-GSG4-P complex shows the phosphate group poised to attack the glycosidic bond and promote phosphorolysis. In all three complexes the pentasaccharide exhibits an altered conformation across sub-sites -1 and +1, the site of catalysis, from the preferred conformation for alpha(1-4)-linked glucosyl polymers. (+info)
Interaction with amylopectin influences the ability of granule-bound starch synthase I to elongate malto-oligosaccharides.
(15/1049)
This paper examines the properties in soluble form of two isoforms of starch synthase. One of these, granule-bound starch synthase I (GBSSI), is responsible for the synthesis of amylose inside the amylopectin matrix of the starch granule in vivo. The other, starch synthase II (SSII), is involved in amylopectin synthesis. Both isoforms can use amylopectin and malto-oligosaccharide as substrates in vitro. As well as acting as a substrate for GBSSI, amylopectin acts as an effector of this isoform, increasing the rate at which it elongates malto-oligosaccharides and promoting a processive rather than distributive mode of elongation of these compounds. The affinity of GBSSI for amylopectin as an effector is greater than its affinity for amylopectin as a substrate. The rate and mode of elongation of malto-oligosaccharides by SSII are not influenced by amylopectin. These results suggest that specific interaction with amylopectin in the matrix of the starch granule is a unique property of GBSSI and is critical in determining the nature of its products. (+info)
Roles of catalytic residues in alpha-amylases as evidenced by the structures of the product-complexed mutants of a maltotetraose-forming amylase.
(16/1049)
The crystal structures of the four product-complexed single mutants of the catalytic residues of Pseudomonas stutzeri maltotetraose-forming alpha-amylase, E219G, D193N, D193G and D294N, have been determined. Possible roles of the catalytic residues Glu219, Asp193 and Asp294 have been discussed by comparing the structures among the previously determined complexed mutant E219Q and the present mutant enzymes. The results suggested that Asp193 predominantly works as the base catalyst (nucleophile), whose side chain atom lies in close proximity to the C1-atom of Glc4, being involved in the intermediate formation in the hydrolysis reaction. While Asp294 works for tightly binding the substrate to give a twisted and a deformed conformation of the glucose ring at position -1 (Glc4). The hydrogen bond between the side chain atom of Glu219 and the O1-atom of Glc4, that implies the possibility of interaction via hydrogen, consistently present throughout these analyses, supports the generally accepted role of this residue as the acid catalyst (proton donor). (+info)