Metabolism, autotrophic 1-carbon

The question is therefore, what are the principal requirements of an autotrophic carbon-fixation mechanism An organic molecule serves as a C02 acceptor molecule, which becomes carboxylated by a carboxylase enzyme. This C02 acceptor molecule needs to be regenerated in a reductive autocatalytic cycle. The product that can be drained off from such a metabolic cycle should be a central cellular metabolite, from which all cellular building blocks for polymers can be derived examples of such central metabolites are acetyl-CoA, pyruvate, oxaloacetate, 2-oxoghitarate, phosphoe-nolpyruvate, and 3-phosphoglycerate. Importantly, the intermediates should not be toxic to the cell. The irreversible steps of the pathway are driven by ATP hydrolysis, while the reduction steps are driven by low-potential reduced coenzymes. 

According to a general rule of organic chemistry, reactions involving the smallest molecules are catalytically the most restrictive. This rule holds notably for the build-up of carbon skeletons with the arithmetic Cl -F Cl = C2 (e.g., C2 = glycine or acetyl thioester). Therefore, it may not come as a surprise that in the course of metabolic evolution, these most simple carbon fixation reactions may fall by the wayside. Under these conditions, an autotrophic carbon fixation metabolism can only be maintained by a metabolic cycle, which multiplies the C2 unit autocatalytically in the absence of its de novo synthesis. A prominent example is the reductive citric acid cycle (C2 -F Cl... 

Carbon dioxide is produced as a result of metabolism of all heterotrophic organisms. The concentrations of CO2 in pore water of reduced sediments are therefore high. Autotrophic microorganisms consume CO2 in the oxidized part of the sediment, which can vary in depth from a meter in deep sea sediments to a few mm... 

Composting is a biological process mediated by microbes belonging to the kingdom Protest, which includes bacteria, algae, fungi, protozoa, and virus particles (Table 12.2). Microbes can be classified into metabolic types based on the carbon and energy sources utilized by the cell. Autotrophs use carbon dioxide as a... 

Plants must be especially versatile in their handling of carbohydrates, for several reasons. First, plants are autotrophs, able to convert inorganic carbon (as C02) into organic compounds. Second, biosynthesis occurs primarily in plastids, membrane-bounded organelles unique to plants, and the movement of intermediates between cellular compartments is an important aspect of metabolism. Third, plants are not motile they cannot move to find better supplies of water, sunlight, or nutrients. They must have sufficient metabolic flexibility to allow them to adapt to changing conditions in the place where they are rooted. Finally, plants have thick cell walls made of carbohydrate polymers, which must be assembled outside the plasma membrane and which constitute a significant proportion of the cell s carbohydrate. 

Woodward, J.R. In Advances in Autotrophic Microbiology and One Carbon Metabolism. Editors Codd, G.A., Dijkhuizen, L. and Tabita, F.R. Kluwer Academic Publishers, Dordrecht, Netherlands,1990, Vol. 1, pp 193-225. 

Some versions of metabolism first schemes have drawn criticism in the literature.71,72 The schemes have assumed that a self-sustaining metabolic network must be autotrophic 73 that is, it must obtain its carbon supply entirely from carbon dioxide, rather than using carbon compounds made by abiotic processes (Sections 5.2.1 and 5.2.2). The assumption that chemicals in proposed reactions in the central metabolic cycle will be catalyzed by other compounds that participate in the cycle has also been questioned.74 Such features are attractive but not essential to the idea of life with small molecules. It has been argued that the objections can be met in principle by introducing a small number of assumptions 75... 

The carbon required by living organisms is an important constituent of cellular structure and metabolic compounds. This element is present in the environment in many forms. It may appear in simple form as gaseous carbon dioxide or as more complex organic compounds. Microorganisms are remarkably diverse in their carbon requirements and they are divided into two groups autotrophs and heterotrophs, based on their carbon source. 

It is likely that the use of CO remains from the early atmospheric conditions when life first evolved around 4 billion years ago. This follows from the hypothesis that the first organisms were autotrophic (Huber and Wachtershauser, 1997 Russell et al., 1998). Volcanic gases can contain as high as 1% CO. Early life forms evolving in volcanic sites or hydrothermal vents could have used CO as their carbon and energy source. If this scenario is correct, CO metabolism today can be viewed as the extant survivor of early metabolic processes (Huber and Wachtershauser, 1997). The ability to metabolize CO is still important today since about 10 tons of CO are removed from the lower atmosphere of the earth by bacterial oxidation every year (Bartholomew and Alexander, 1979). This helps to maintain CO below toxic levels, except in extreme cases. 

The catalysis of the transfer of a methyl group is an important role of enzyme-bound vitamin B12 derivatives in human, animal, and bacterial metabolism. The known enzyme-controlled methyl group-transfer reactions are key steps in the cobamide-dependent methylations of homocysteine to methionine, in the metabolic formation of methane from other Ci-compounds in methanogenic bacteria, and in the fixation of carbon dioxide via the acetyl coenzyme A pathway of some bacterial autotrophs (Figure 10). ... 

The autotrophic NH3 oxidizers are considered to be obligate chemolithoautotrophs with no source of energy other than NH3 and no net source of cellular carbon other than CO2. Obhgate autotrophy had been attributed to the absence of one or more enzymes in the tricarboxyhc acid cycle, but inspection of the complete genome for Nitrosomonas europaea shows that a complete TCA cycle is present (Chain et al., 2003). N. europaea has hmited genetic capabihty for transport and metabolism of organic molecules, but the basis for its apparent obhgate dependence upon NH4 and CO2 is still not entirely clear. 

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