Mechanism of maltal hydration catalyzed by beta-amylase: role of protein structure in controlling the steric outcome of reactions catalyzed by a glycosylase.

Crystalline (monomeric) soybean and (tetrameric) sweet potato beta-amylase were shown to catalyze the cis hydration of maltal (alpha-D-glucopyranosyl-2-deoxy-D-arabino-hex-1-enitol) to form beta-2-deoxymaltose. As reported earlier with the sweet potato enzyme, maltal hydration in D2O by soybean beta-amylase was found to exhibit an unusually large solvent deuterium kinetic isotope effect (VH/VD = 6.5), a reaction rate linearly dependent on the mole fraction of deuterium, and 2-deoxy-[2(a)-2H]maltose as product. These results indicate (for each beta-amylase) that protonation is the rate-limiting step in a reaction involving a nearly symmetric one-proton transition state and that maltal is specifically protonated from above the double bond. This is a different stereochemistry than reported for starch hydrolysis. With the hydration catalyzed in H2O and analyzed by gas-liquid chromatography, both sweet potato and soybean beta-amylase were found to convert maltal to the beta-anomer of 2-deoxymaltose. That maltal undergoes cis hydration provides evidence in support of a general-acid-catalyzed, carbonium ion mediated reaction. Of fundamental significance is that beta-amylase protonates maltal from a direction opposite that assumed for protonating starch, yet creates products of the same anomeric configuration from both. Such stereochemical dichotomy argues for the overriding role of protein structures in dictating the steric outcome of reactions catalyzed by a glycosylase, by limiting the approach and orientation of water or other acceptors to the reaction center.     

Design of a growth model for yeast

The study of the cell cycle of a yeast strain made it possible to define two parameters:T, the time elapsing between the appearance of two consecutive buds on a mother cell, and Θ, the time elapsing between the appearance of a bud and the beginning of the first mitotic cycle. The influence of these two parameters on the growth rate of the strain is studied.     

A new species of bound ubisemiquinone anion in QH2: cytochrome c oxidoreductase

Using a combination of EPR and low temperature diffuse reflectance spectroscopy, a new species of semiquinone anion has been detected in QH2:cytochrome c oxidoreductase in submitochondrial particles under conditions of oxidant-induced extra reduction of cytochrome b. In contrast to the previously detected semiquinone anion, this new species is insensitive to antimycin but sensitive to treatment with 2,3-dimercaptopropanol and O2. The two species can easily be distinguished on the basis of their respective EPR properties since they differ in g-value, line width, and microwave power saturation behavior. It is concluded that the two species of semiquinone anion are bound to different domains on QH2:cytochrome c oxidoreductase. The existence of two different semiquinone anions in the enzyme strongly supports a mechanism of electron flow as proposed in the Q-cycle.     

Study of two cases of ring 13 chromosome using high-resolution banding.

The chromosomes of two patients with ring 13 (r13) were studied using high-resolution RBG banding of prometaphase cells. The rings of the two patients differ slightly in breakpoints. Cell with multiple single, double-sized rings, quadruple-sized rings, rod- and ring-shaped fragments, and fragments showing varied states of condensation were seen, as were cells monosomic for chromosome 13. The evolution of these cell lines as a result of sister chromatid exchange, nondisjunction, ring breakage, and premature chromosome condensation is discussed. Clinical features of these patients reflect the heterogeneity of phenotype for r13 patients. Each case includes a feature of trisomy 13. The significance of mosaicism of cell lines in patients bearing ring chromosomes is considered with respect to variation in clinical findings.     

Liver mitochondria, confirmed as intact by complete suppression of succinate uptake and oxidation, possess a carnitine palmitoyltransferase I that is totally inhibited by malonyl CoA.

Succinate dehydrogenase activity in mitochondria, which were isolated by centrifuging partially purified mitochondria through 1. 315 M sucrose, was completely suppressed when [14C]succinate uptake was abolished by prior incubation of the mitochondria with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and valinomycin. The conclusion that these mitochondria were intact was confirmed by the fact that, when these mitochondria were broken by a freeze-thaw cycle followed by sonication, such inhibition was totally abolished. The yield of mitochondria, microsomes, and peroxisomes from the initial homogenate was 17.8, <0.1, and 0%, respectively, indicating that the mitochondria were not only intact but also essentially free of contamination from microsomes and peroxisomes. The overt form of carnitine palmitoyltransferase (CPT I) in these intact and pure mitochondria was totally inhibited by malonyl CoA, indicating that previous reports of incomplete inhibition in mitochondrial preparations resulted from interference from CPT activity in the inner mitochondrial membrane (CPT II), microsomes, or peroxisomes.