Autotrophic pathway

Ljungdahl LG. 1986. The autotrophic pathway of acetate synthesis in acetogenic bacteria. Annu Rev Microbiol 40 415-50. 

Reaction 2 above shows the reductive formation of acetate from CO2. This occurs via the acetyl-CoA pathway so called beeause the acetyl group of acetyl-CoA is the first 2-carbon moiety in which both carbons originate from CO2. The acetyl-CoA pathway is now recognized as an autotrophic pathway of CO2 fixation. The pathway is outlined in Fig. 1. Carbon dioxide enters the pathway via two reductive reactions. One, catalyzed by an NADP-dependent formate hydrogenase (FDH), leads to the formation of formate that, in a series of reaetions involving tetrahydrofolate (THF)intermediates is reduced to methyl-THF, the methyl group of which is... 

Ljungdahl LG (1986) The autotrophic pathway of acetate synthesis in acetogenic bacteria. Ann Rev Microbiol 40 415—450 Ljungdahl LG (1994) The acetyl-CoA pathway and the chemiosmotic generation of ATP during acetogenesis. In Drake HL (ed) Acetogenesis. Chapman and Hall, New York, pp 63-85... 

A strain of Rhodopseudomonas palustris, which was isolated by enrichment with taurine, could use this as electron source, and as a source of sulfur and nitrogen during photo-autotrophic growth with CO2. Taurine was metabolized to sulfoacetaldehyde and acetyl phosphate by a pathway, which has already been noted (Novak et al. 2004). 

The theory is based on the autotrophic metabolism of low-molecular-weight constituents in an environment of iron sulfide and hot vents. Figure 2.4 gives an illustration of one reaction pathway. It is worthwhile to consider that the metabolism is a surface metabolism, namely with a two-dimensional order, based on negatively charged constituents on a positively charged mineral surface. Actually Wachtershauser sees this as an interesting part of a broader philosophical view (Huber and Wachtershauser, 1997). 

Outline the pathway for biosynthesis of L-leucine from glucose and NH4+ in autotrophic organisms. In addition, outline the pathways for degradation of leucine to C02, water, and NH4+ in the human body. For this overall pathway or "metabolic loop," mark the locations (one or more) at which each of the following processes occurs. 

Cysteine is formed in plants and in bacteria from sulfide and serine after the latter has been acetylated by transfer of an acetyl group from acetyl-CoA (Fig. 24-25, step f). This standard PLP-dependent (3 replacement (Chapter 14) is catalyzed by cysteine synthase (O-acetylserine sulfhydrase).446 447 A similar enzyme is used by some cells to introduce sulfide ion directly into homocysteine, via either O-succinyl homoserine or O-acetyl homoserine (Fig. 24-13). In E. coli cysteine can be converted to methionine, as outlined in Eq. lb-22 and as indicated on the right side of Fig. 24-13 by the green arrows. In animals the converse process, the conversion of methionine to cysteine (gray arrows in Fig. 24-13, also Fig. 24-16), is important. Animals are unable to incorporate sulfide directly into cysteine, and this amino acid must be either provided in the diet or formed from dietary methionine. The latter process is limited, and cysteine is an essential dietary constituent for infants. The formation of cysteine from methionine occurs via the same transsulfuration pathway as in methionine synthesis in autotrophic organisms. However, the latter use cystathionine y-synthase and P-lyase while cysteine synthesis in animals uses cystathionine P-synthase and y-lyase. 

Aromatic compounds arise in several ways. The major mute utilized by autotrophic organisms for synthesis of the aromatic amino acids, quinones, and tocopherols is the shikimate pathway. As outlined here, it starts with the glycolysis intermediate phosphoenolpyruvate (PEP) and erythrose 4-phosphate, a metabolite from the pentose phosphate pathway. Phenylalanine, tyrosine, and tryptophan are not only used for protein synthesis but are converted into a broad range of hormones, chromophores, alkaloids, and structural materials. In plants phenylalanine is deaminated to cinnamate which yields hundreds of secondary products. In another pathway ribose 5-phosphate is converted to pyrimidine and purine nucleotides and also to flavins, folates, molybdopterin, and many other pterin derivatives. 

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