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PubMed1976

Factors affecting the activity of citrate synthase of Acetobacter xylinum and its possible regulatory role.

The citrate synthase activity of Acetobacter xylinum cells grown on glucose was the same as of cells grown on intermediates of the tricarboxylic acid cycle. The activity of citrate synthase in extracts is compatible with the overall rate of acetate oxidation in vivo. The enzyme was purified 47-fold from sonic extracts and its molecular weight was determined to be 280000 by gel filtration. It has an optimum activity at pH 8.4. Reaction rates with the purified enzyme were hyperbolic functions of both acetyl-CoA and oxaloacetate. The Km for acetyl-CoA is 18 mum and that for oxaloacetate 8.7 mum. The enzyme is inhibited by ATP according to classical kinetic patterns. This inhibition is competitive with respect to acetyl-CoA (Ki = 0.9 mM) and non-competitive with respect to oxaloacetate. It is not affected by changes in pH and ionic strength and is not relieved by an excess of Mg2+ ions. Unlike other Gram-negative bacteria, the A. xylinum enzyme is not inhibited by NADH, but is inhibited by high concentrations of NADPH. The activity of the enzyme varies with energy charge in a manner consistent with its role in energy metabolism. It is suggested that the flux through the tricarboxylic acid cycle in A. xylinum is regulated by modulation of citrate synthase activity in response to the energy state of the cells.

Acetyl Coenzyme A↗
PubMed1978

Purification and regulatory properties of the oxaloacetate decarboxylase of Acetobacter xylinum.

The oxaloacetate (OAA) decarboxylase (EC 4.1.1.3) activity of Acetobacter xylinum cells grown on glucose or glycerol is the same as that of cells grown on intermediates of the citrate cycle. The enzyme was purified 92-fold from extracts, and its molecular weight was determined to be 100,000 by gel filtration. Initial velocity studies revealed marked positive cooperativity for OAA (Hill coefficient [n(H)] = 1.8; S(0.5) = 21 mM). The affinity of the enzyme for OAA was markedly increased upon addition of nicotinamide adenine dinucleotide (NAD), NAD phosphate (NADP), and some other pyridine nucleotides. S(0.5(OAA)) decreased to 1 mM but n(H) and V(max) were unchanged. Saturation kinetics for the pyridine nucleotides were hyperbolic, and a half-maximal effect was obtained with 8 muM NAD and 30 muM NADP. The enzyme also catalyzed the exchange of (14)CO(2) into OAA but not the net carboxylation of pyruvate. Exchange activity, too, exhibited sigmoidal kinetics for OAA and was strongly stimulated by NAD at low substrate concentrations. The enzyme was inhibited by acetate competitively with respect to OAA. The K(I) for acetate (12 mM) was well within the physiological range of this compound inside the cell. The regulatory properties of the decarboxylase with respect to OAA cooperativity, NAD activation, and acetate inhibition were retained in situ within permeabilized cells. These properties seem to provide for a control mechanism which could insure the maintenance of OAA and the citrate cycle during growth of cells on glucose and, conversely, the required supply of pyruvate during growth on intermediates of the citrate cycle.

Carbon Dioxide↗
PubMed1977

The structure of cellulose-producing bacteria, Acetobacter xylinum and Acetobacter acetigenus.

The structure of the pellicles and cells of the cellulose-producing bacteria, Acetobacter xylinum and Acetobacter acetigenus, was studied by transmission electron microscopy of thin sections and freeze-etch replicas of glucose-stimulated cell suspensions, quiescent cell suspensions, and discrete pellicles. These bacteria have a relatively thin cell wall in section, with several irregular features superimposed on an otherwise simple, Gram-negative morphology. There are no flagella or pili. Unfixed, unextracted cells, viewed as whole mounts, show spherical or ellipsoidal bodies of undetermined composition which disappear after extraction with water or ethanol and propylene oxide. For both species, there are several kinds of cell surface irregularities, some of which are localized protrusions of the cell envelope. A variety of irregularities is seen frequently on cells in the first minutes of glucose incubation, on cells in a discrete pellicle, on quiescent cells, and on starved cells. Immediately after the addition of glucose to cellulose-free cells in suspension culture, fine fibrils appear on and (or) near the cell envelope. The fine fibrils are frequently as small as 3 nm in diameter in both freeze-etch and thin-section preparations and are frequently associated with freshly synthesized cellulose fibrils. Starved cells in suspensions free of (classical) microfibrils sometimes reveal stubs of an extracellular structure whose morphology resembles that of a nascent cellulose fibril.

Cell Membrane↗
PubMed1977

Alpha-ketoglutarate dehydrogenase complex of Acetobacter xylinum. Purification and regulatory properties.

The alpha-ketoglutarate dehydrogenase complex of Acetobacter xylinum was purified to homogeneity. It consists of three main polypeptide chains with a total molecular weight of about 2.4 X 10(6). It catalyzes the overall Mg2+ and thiamin pyrophosphate-dependent, NAD+- and CoA-linked oxidative decarboxylation of alpha-ketoglutarate, as well as the partial reactions characteristic of the three enzyme components described for the complex from other sources. Initial velocity studies revealed marked positive cooperativity for the substrate alpha-ketoglutarate (Hill coefficient (nH) = 2.0; concentration of ligand at half-maximum effect (S0.5) = 8 mM). The sigmoidal [alpha-ketoglutarate]-velocity relationship became hyperbolic upon addition of AMP or 3-acetylpyridine adenine dinucleotide (AcPyAD) or in the presence of high concentrations of NAD. S0.5 (alpha-ketoglutarate) decreased to 1 mM, but Vmax was unchanged. Saturation curves for NAD and AMP are sigmoidal (nH = 2) at low alpha-ketoglutarate concentrations and become hyperbolic at high alpha-ketoglutarate concentrations. As judged by S0.5, the relative efficiency of the allosteric effectors is AcPyAD greater than AMP greater than alpha-ketoglutarate- greater than NAD+. Half-maximal changes in nH, S0.5, and activation by AMP occur at a pH significantly different from that of half-maximal activity. A model for the allosteric behavior of the complex is proposed in which the first enzyme component of the complex (E1) is the site for the allosteric interactions and AMP is the primary positive modifier, whereas NAD and AcPyAD act as AMP analogues. The overall reaction is competitively inhibited by NADH with respect to NAD (K1 = 20 micronM) and by succinyl-CoA with respect of CoA (K1 = 3 micronM). The properties of the alpha-ketoglutarate dehydrogenase complex of A. xylinum appear to provide for appropriate partitioning of alpha-ketoglutarate carbon between competing pathways in response to the energy state of the cells.

Adenosine Monophosphate↗
PubMedDate not supplied

[Remarks on ethanol oxidation by an "Acetobacter xylinum" microbial electrode (author's transl)].

A "microbial electrode" for ethanol assay has been designed using combination of an oxygen probe and cellulosic pellicle of Acetobacter xylinum. Assay is feasible with an ethanol concentration below 0.4 mM on a pH range of 2,5-7. The formation of acetic acid leds to a competitive inhibition of ethanol oxidation as observed with free cells. Pellicle stability at room temperature is good over a ten hours period. At 4 degrees C, film preservation is quite satisfactory over a ten days storage period. The author compares the ethanol oxidation kinetics observed, using both cellulosic pellicles and free cells of A. xylinum.

Acetaldehyde↗