Volume 87, Issue 4
April 1978, pages 81-145
pp 81-94 April 1978 Biochemistry
Aspartate transcarbamylase (EC 220.127.116.11) catalyzes the bi substrate reaction—carbamyl phosphate+ L-aspartate ⇌ carbamyl aspartate ⇌ phosphate, The order of addition of substrates and release of products for the homogeneous aspartate transcarbamylase fromPhaseolus aureuss eedlings has been investigated by using the kinetic methods of analysis. p ]Initial velocity studies indicated that the mechanism might be a sequential one. Product inhibition studies showed that phosphate was a linear competitive inhibitor with respect to carbamyl phosphate and was anS (slope) andI (intercept) linear noncompetitive inhibitor with respect to aspartate. Carbamyl aspartate was a noncompetitive inhibitor with respect to both the substrates. These inhibition patterns agreed with an ordered mechanism of reaction with carbamyl phosphate as the leading substrate and phosphate as the last product to leave the enzyme surface. The presence of dead end complexes and the rapid equilibrium random mechanism were ruled out by the absence of inhibition by the substrate(s) and the linear replot slopevs. the inhibitor concentration.
Acetyl phosphate, an analog ue of carbamyl phosphate was a non-competitive inhibitor with respect to aspartate. This result could be explained both in terms of an ordered as well as a random mechanism. On the other hand, succinate, an analog ue of aspartate was an uncompetitive inhibitor with respect to carbamyl phosphate, indicating that the mechanism was ordered. p ]The transition state analog ue, N-(phosphonoacetyl)-L-aspartate, binds much more tightly than either of the two substrates. This analog ue was a linear competitive inhibitor with respect to carbamyl phosphate and a linear noncompetitive inhibitor with respect to aspartate. These results are compatible with an ordered mechanism rather than a random one.
pp 95-108 April 1978 Biochemistry
An early event in the temporal sequence of molecular processes that occur during cytokinin and/or K+- induced growth and differentiation of etiolated cotyledons ofCucumis sativus L. var. Guntur in culture, was an enhanced specific permeability, resulting in increased uptake of32P043− and 2-deoxy-[3H]-D-glucose. Concomitant increases were also noticed in RNA synthesis followed by enhanced elaboration of protein, DNA and chlorophyll in that order. However, the cytokinins (benzyladenine or benzyladenosine) did not increase cyclic AMP concentrations or the preferential synthesis of poly(A)-rich RNA. K+ augmented [3H]-benzyladenine uptake thereby explaining the synergistic action of the two growth promoters. [3H]-Benzyladenine was metabolized in this tissue presumably to benzyladenosine and benzyladenosine monophosphate, It is suggested that the early alteration in the permeability of the tissue induced by cytokinin and/or K+ results in a greater mobilization and utilization of the stored products in the cotyledon culminating in growth and differentiation into a leaf-like structure.
pp 109-118 April 1978 Biochemistry
The effect of glucose on the respiratory metabolism of two facultative anaerobesSaccharomyces cerevisiae andEscherichia coli has been studied and a close similarity is reported. In both cases, there is disorganisation of the membrane bound electron transport components.
pp 119-126 April 1978 Biochemistry
Isocitrate lyase (Ee 4·1,3·1) was purified seventy fold from gamma irradiated banana pulp tissue acetone powder. It showed an optimum pH of 6·0, and the Km value for DL-isocitrate was 0·8 mM. Among the various metabolic inhibitors, oxaloacetate was found to be the most potent and its inhibition was competitive. The enzyme activity was not dependent on externally added Mg2+. The Mg2+ content of the purified enzyme was 10–12 ng/rng protein. A method for the detection of the two multiple forms of isocitrate lyase present in this preparation was developed using 2,4-dinitrophenylhydrazine as detecting agent for glyoxylate formed during the isocitrate lyase reaction.
pp 127-133 April 1978 Microbiology and Cell Biology
The effects of two amino acid analogues, viz., L-methionine-DL-sulphoximine and L-methyl-DL-methionine on growth, heterocyst differentiation and nitrogen fixation in the blue-green algaNostoc linckia have been studied with special reference to heterocyst spacing pattern. L-methionine-DL-sulphoximine strongly inhibited growth but produced an unusual number of heterocysts with changed heterocyst spacing pattern in both nitrogen-free and ammonium-containing media. L-methyl-DL-methionine was less effective than L-methionine-DL-sulphoximine. An attempt was also made to counteract the toxic effects of these analogues by supplying amino acids. Glutamine and methionine reversed the inhibitory effect of L-methionine-DL-sulphoximine while only methionine reversed the inhibitory effect of L-methyl-DL-methionine. Production of changed heterocyst spacing pattern in nitrogen-free and ammonium-containing media when supplemented with L-methionine-DL-sulphioximine suggests that ammonia may not be the inhibitor of heterocyst spacing pattern.
pp 135-145 April 1978 Biophysics
Hydrogen bonding contributes of the order of 5–15 kcal/mol base pair to the stability of the helix (electronic or intrinsic energy). This contribution is selective, i.e., there is a preferential stability of the Watson-Crick G-C pair relative to all other pairs. Stacking interactions contribute approximately of the same order as hydrogen bonding. Perhaps the most interesting aspect of the stacking interactions which emerges from the theoretical analysis is the fact that the stacking maxima are not necessarily at the angles the successive base pair plans assume in a regular double helix. Consequently some sequence dependent structure peculiarities may arise. That is, the double helix may have a fine structure contingent on the sequence of base pairs. Indeed such sequence dependent polymorphism has been reported in the recent literature and appears to influence the ability of aromatic drugs to intercalate into the helix. The solvent effect which is another factor of stability seems to decrease somewhat bonding scheme preferences. For example, in the model we used to estimate solvent effect, we find that the G-C pair formation is de-stabilized strongly in water, while the A-T pair formation is mildly enhanced. The continuum model of solvent effect leads to similar qualitative conclusions. Studies of backbone conformation indicate that only a limited range of conformational states are comparable with the helical configuration. Improved empirical methods are needed in order to successfully calculate backbone effects for relatively large segments of nucleic acids.