Structural basis of the thermostability of monomeric malate synthase from a thermophilic Bacillus

Comparative thermodynamic elucidation of the structural stability of thermophilic proteins

Differential scanning calorimetry, circular dichroism, and visible absorption spectrophotometry were employed to elucidate the structural stability of thermophilic phycocyanin derived from Cyanidium caldarium, a eucaryotic organism which contains a nucleus, grown in acidic conditions (pH 3.4) at 54 degrees C. The obtained results were compared with those previously reported for thermophilic phycocyanin derived from Synechococcus lividus, a procaryote containing no organized nucleus, grown in alkaline conditions (pH 8.5) at 52 degrees C. The temperature of thermal unfolding (t(d)) was found to be comparable between C. caldarium (73 degrees C) and S. lividus (74 degrees C) phycocyanins. The apparent free energy of unfolding (DeltaG([urea]=0)) at zero denaturant (urea) concentration was also comparable: 9.1 and 8.7 kcal/mole for unfolding the chromophore part of the protein, and 5.0 and 4.3 kcal/mole for unfolding the apoprotein part of the protein, respectively. These values of t(d) and DeltaG([urea]=0) were significantly higher than those previously reported for mesophilic Phormidium luridum phycocyanin (grown at 25 degrees C). These findings revealed that relatively higher values of t(d) and DeltaG([urea]=0) were characteristics of thermophilic proteins. In contrast, the enthalpies of completed unfolding (DeltaH(d)) and the half-completed unfolding (DeltaH(d)) 1 2 for C. caldarium phycocyanin were much lower than those for S. lividus protein (89 versus 180 kcal/mole and 62 versus 115 kcal/mole, respectively). Factors contributing to a lower DeltaH(d) in C. caldarium protein and the role of charged groups in enhancing the stability of thermophilic proteins were discussed.

Regulation of isocitrate dehydrogenase by phosphorylation in Escherichia coli K-12 and a simple method for determining the amount of inactive phosphoenzyme

In several Escherichia coli K-12 strains grown on a limiting concentration of glucose, isocitrate dehydrogenase (IDH) was inactivated about 90% after cessation of growth upon exhaustion of the glucose. Such inactivation has been previously observed in several E. coli strains but not in E. coli K-12 (unless acetate was added to the bacterial culture when growth ceased). IDH was inactivated 75 to 80% in all E. coli K-12 strains we examined during growth on acetate. The inactivation involved phosphorylation of the enzyme and is considered to be a regulatory mechanism facilitating metabolite flow along the glyoxylate shunt. Phospho-IDH interacted with antibodies to enzymatically active IDH. We have devised a method, based on this immunological cross-reaction, for determining the proportions of active and inactive (phospho-) IDH in cell extracts.

Malate synthase: aggregation, deaggregation, and binding of phospholipids

Octameric malate synthase is located in the glyoxysomes of cucumber cotyledons. The enzyme is predominantly confined to the organelle's membrane and can be solubilized with Mg2+. Separation of cell structures in a zonal rotor afforded, besides glyoxysomes, two other zones with malate synthase activity, viz., in the gradient supernatant and in the range of the endoplasmic reticulum (ER). Malate synthases of these three fractions were purified to apparent homogeneity and classified according to their molecular weight. Differences in subunit molecular weight, however, could not be detected when malate synthases from the three fractions were compared. Mature malate synthase, as well as malate synthase prepared from fractions sedimenting similarly to the ER, exhibited the following behavior with respect to aggregation and deaggregation: at low salt concentrations and in the absence of Mg2+, the enzyme shifted to aggregated forms (approx 100 S); with 2 mM Mg2+, malate synthase deaggregated and occurred predominantly as an octamer (19 S). By changing buffer conditions, mature forms of malate synthase could be interconverted repeatedly between octameric and aggregated forms, whereas a monomeric form (5 S), prepared from soluble fractions assigned to the cytosol, did not oligomerize. The amphipathic properties of malate synthase were demonstrated by the enzyme's capacity for binding phospholipids.

Purification and some properties of an extracellular protease (caldolysin) from an extreme thermophile

An extracellular metal-chelator-sensitive lytic protease (assigned the trivial name caldolysin) was isolated from a Thermus-like organism, Thermus T-351. Caldolysin was purified by affinity chromatography on Cbz-D-phenylalanine-TETA-Sepharose 4B and by gel filtration. It contained 13% carbohydrate, a single zinc atom, had a molecular weight of approx. 21,000, a pH optimum of 8 (azocasein substrate), and an isoelectric point of about 8.5. It was capable of hydrolysing many soluble and insoluble protein substrates, including collagen and elastin. No esterase activity was detected, and small peptides (less than four amino acids) and low molecular weight chromogenic substrates were not hydrolysed. A specificity for small aliphatic amino acids on either side of the splitting point was indicated. Caldolysin lysed heat-killed Gram-negative bacterial cells, but had little effect on Gram-positive organisms. Caldolysin exhibited a very high degree of thermostability (t 1/2(80 degrees C) approximately 30 h, t 1/2(90 degrees C) = 1 h). The stability (but not activity) was shown to be dependent on the presence of Ca2+ (t 1/2(75 degrees C, 10 mM calcium) greater than 193 h; t 1/2(75 degrees C, no calcium) = 4.8 min). None of the other metal ions tested (Co, Zn, Sr, Mg, Ba and Cu) was as effective as calcium in conferring thermostability of EDTA-treated caldolysin. Caldolysin was stable at room temperature in moderately acid and alkaline (pH 5 to 11) buffers for periods of greater than 90 days. Little loss of enzyme activity was detected after the incubation of caldolysin at 18 degrees C in the presence of 8 M urea, 6 M guanidine hydrochloride or 1% sodium dodecyl sulphate for 24 h. At 75 degrees C, the activity half-life of caldolysin in these denaturing agents was reduced to approx. 1 h, 1 h and more than 5 h, respectively.

The purification and some properties of a stereospecific D-asparaginase from an extremely thermophilic bacterium, Thermus aquaticus

A specific D-asparaginase was isolated and crystallized from Thermus aquaticus strain T351. It is present in larger amounts than the L-asparaginase. The enzyme has a molecular weight of 60 000, an isoelectric point of 4.8 and a Km of 2 mM. It has 6 disulphide bonds/molecule, and a histidine residue at the active site. It is inhibited by keto acids and by high salt concentrations.