The acetyl-CoA pathway of autotrophic growth

Abstract The most direct conceivable route for synthesis of multicarbon compounds from CO₂ is to join two molecules of CO₂ together to make a 2-carbon compound and then polymerize the 2-carbon compound or add CO₂ successively to the 2-carbon compound to make multicarbon compounds. Recently, it has been demonstrated that the bacterium, Clostridium thermoaceticum , grows autotrophically by such a process. The mechanism involves the reduction of one molecule of CO₂ to a methyl group and then its combination with a second molecule of CO₂ and CoA to form acetyl-CoA. We have designated this autotrophic pathway the acetyl-CoA pathway [1]. Evidence is accumulating that this pathway is utilized by other bacteria that grow with CO₂ and H₂ as the source of carbon and energy. This group includes bacteria which, like C. thermoaceticum , produce acetate as a major end product and are called acetogens or acetogenic bacteria. It also includes the methane-producing bacteria and sulfate-reducing bacteria.
The purpose of this review is to examine critically the evidence that the acetyl-CoA pathway occurs in other bacteria by a mechanism that is the same or similar to that found in C. thermoaceticum . For this purpose, the mechanism of the acetyl-CoA pathway, as found in C. thermoaceticum , is described and hypothetical mechanisms for other organisms are presented based on the acetyl-CoA pathway of C. thermoaceticum . The available data have been reviewed to determine if the hypothetical schemes are in accord with presently known facts. We conclude that the formation of acetyl-CoA by other acetogens, the methanogens and sulphate-reducing bacteria occurs by a mechanism very similar to that of C. thermoaceticum .     

Inhibition of exogenous NADH oxidation in plant mitochondria by chlorotetracycline in the presence of calcium ions

Chlorotetracycline inhibits the uncoupled oxidation of exogenous NADH by Jerusalem artichoke (Helianthus tuberosus L.) mitochondria extensively (over 80%) and rapidly (inhibition complete in 10 s) in the presence of added Ca²⁺. Half-maximal inhibition is observed at 15 μM chlorotetracycline in the presence of 2 mM Ca²⁺. The oxidation of succinate is only affected marginally by chlorotetracycline plus Ca²⁺. The inhibition of NADH oxidation and the fluorescence of CTC are well correlated. Mn²⁺ is the only other cation which shows an (increased) inhibition in the presence of chlorotetracycline. The inhibition by Ca²⁺ and chlorotetracycline disappears at acid pH, and the pH optimum in their presence is 6.4. The inhibition caused by other lipid-soluble Ca²⁺-chelators is not reversible or is enhanced by the addition of excess Ca²⁺. In contrast, inhibition caused by relatively water-soluble chelators is completely reversed by added Ca²⁺. It is suggested that a neutral 1:2 complex is formed between Ca²⁺ and chlorotetracycline which can substitute for Ca²⁺ bound at sites in the lipophilic phase of the inner mitochondrial membrane, which are essential for the activity of the external NADH dehydrogenase.     

Streptozotocin-induced diabetes produces alterations in adenylate cyclase in rat cerebrum, cerebral microvessels and retina

Eight weeks following streptozotocin-induced diabetes mellitus in rats, the sensitivity of adenylate cyclase to dopamine (DA) and norepinephrine (NE) was reduced in homogenates of retina. Furthermore, the activation of adenylate cyclase in cerebral microvessels (capillaries) by NE, 5'-guanylyl imidodiphosphate (alone or with NE) and forskolin was reduced in diabetic rats versus appropriate controls. In diabetic rats enzyme sensitivity to only NE was attenuated in homogenates of cerebral cortex and cortical piaarachnoid. No differences between controls and diabetics were noted with respect to guanylate cyclase or cyclic AMP phosphodiesterases. The damage observed in retina and microvessels may play an important pathogenic role in diabetes-induced blindness and stroke.     

The diabetic Zucker fatty rat

A noninsulin-dependent diabetes appeared in fatty rats in our Zucker rat colony. A breeding program yielded a genetic pattern of diabetes consistent with a dominant gene not closely linked to the fatty gene. Fatty males were more frequently affected than fatty females. Since no markers could be identified for heterozygous carriers and since affected fatty rats were 6 months old when diabetes appeared, the diabetic trait could not be sustained in our small colony. Glucose tolerance tests showed that the diabetic fatty rats had little increase in plasma insulin concentration after a glucose load was administered. Plasma insulin concentrations were unchanged relative to control fatty rats. Percentage body fat and plasma triglyceride values were decreased in fatty diabetic rats relative to control fatty rats, however, consistent with insulin resistance in fat and liver. The content of pancreatic insulin was markedly decreased in the diabetic fatty rat relative to either the ad libitum or diet-restricted fatty rats. The occurrence of a genetically based diabetes in a normally outbred colony underscores the importance of genetic traits that interact with obesity in determining diabetes in rodent models.     

On the structure of the iron-sulfur complex in the two-iron ferredoxins

Recent spectroscopic and magnetic susceptibility studies of the iron center in the two-iron ferredoxins provide criteria which any model for the iron-sulfur complex in these proteins must satisfy. These criteria are most stringent for parsley and spinach ferredoxin: the reduced proteins contain a high-spin ferric atom antiferromagnetically exchange-coupled (presumably via sulfide bridging ligands) to a high-spin ferrous atom. In the oxidized proteins the iron atoms are antiferromagnetically spin-coupled, high-spin ferric atoms. Arguments are given to substantiate the claim that the ferrous atom in the reduced protein is ligated by four sulfur atoms in a distorted tetrahedral configuration: two are the bridging sulfides, two are cysteinyl sulfurs. A treatment of proton contact shifts based upon the above model is pertinent to proton magnetic resonance data already available and provides a means to identify directly the ligands at both iron atoms via further PMR experiments.     

NUTRITION OF PANDORINA MORUM1,2

A defined medium was developed for 3 strains of Pandorina morum. The strains tested required no vitamins or other organic compounds. The optimal initial pH was between 7.0 and 8.0. Various carbon sources were tested, and only glycolate and acetate appreciably stimulated growth. Mixotrophic growth in the light was stimulated by glycolate in all 3 strains, and by acetate in strains 880 and N76-6. Only strain N76-6 utilized acetate for heterotrophic growth in the dark. Thirty strains of P. morum of world-wide distribution were surveyed for mixotrophic and heterotrophic growth with acetate. All were found to fit 1 of 3 classes with respect to acetate metabolism: (1) no effect in light or dark; (2) stimulation of growth in light only; (3) stimulation of growth in light and dark.