Carbon, in amino acids

The decay constant (T2 = 26 j s) for the resonance at 55 ppm is considerably shorter than expected for methoxy groups but similar to that expected for CH carbons in amino acids. Hence the results suggest a substantial contribution of amino acid carbon to the sample. In contrast, we have observed relatively long decay constants (T = 60 yts) for resonances at similar chemical shifts in terrestrial organic materials which can therefore be assigned to methoxy groups. 

Amino acids differ from carbohydrates and fats in that they contain nitrogen as part of their molecular structure. For the carbons in amino acids to enter into the energy generating metabolic pathways, the amino groups must first be removed so that they can be detoxified and excreted. The amino acid nitrogen is excreted predominantly as urea, but some is also excreted as free ammonia in order to buffer the urine. 

The liver is the major site of amino acid metabolism. It is the major site of amino acid catabolism and converts most of the carbon in amino acids to intermediates of the TCA cycle or the glycolytic pathway (which can be converted to glncose or oxidized to CO2), or to acetyl CoA and ketone bodies. The fiver is also the major site for urea synthesis. It can take up both glutamine and alanine and convert the... 

Glycine is the simplest anino acid and the only one in Table 27.1 that is achiral. The a-carbon atom is a chirality center in all the others. Configurations in amino acids are normally specified by the d, l notational system. All the chiral amino acids obtained from proteins have the l configuration at their- a-carbon atom, meaning that the amine group is at the left when a Fischer projection is arianged so the carboxyl group is at the top. 

In essence, this series of four reactions has yielded a fatty acid (as a CoA ester) that has been shortened by two carbons, and one molecule of acetyl-CoA. The shortened fatty acyl-CoA can now go through another /3-oxidation cycle, as shown in Figure 24.10. Repetition of this cycle with a fatty acid with an even number of carbons eventually yields two molecules of acetyl-CoA in the final step. As noted in the first reaction in Table 24.2, complete /3-oxidation of palmitic acid yields eight molecules of acetyl-CoA as well as seven molecules of FADHg and seven molecules of NADFI. The acetyl-CoA can be further metabolized in the TCA cycle (as we have already seen). Alternatively, acetyl-CoA can also be used as a substrate in amino acid biosynthesis (Chapter 26). As noted in Chapter 23, however, acetyl-CoA cannot be used as a substrate for gluco-neogenesis. 

Fatty acids with odd numbers of carbon atoms are rare in mammals, but fairly common in plants and marine organisms. Humans and animals whose diets include these food sources metabolize odd-carbon fatty acids via the /3-oxida-tion pathway. The final product of /3-oxidation in this case is the 3-carbon pro-pionyl-CoA instead of acetyl-CoA. Three specialized enzymes then carry out the reactions that convert propionyl-CoA to succinyl-CoA, a TCA cycle intermediate. (Because propionyl-CoA is a degradation product of methionine, valine, and isoleucine, this sequence of reactions is also important in amino acid catabolism, as we shall see in Chapter 26.) The pathway involves an initial carboxylation at the a-carbon of propionyl-CoA to produce D-methylmalonyl-CoA (Figure 24.19). The reaction is catalyzed by a biotin-dependent enzyme, propionyl-CoA carboxylase. The mechanism involves ATP-driven carboxylation of biotin at Nj, followed by nucleophilic attack by the a-carbanion of propi-onyl-CoA in a stereo-specific manner. 

The nucleophilic carbon centres of lithium enolates may also be pyrami-dalized, though there is little experimental evidence of this (Seebach et al., 1985, 1991). Silyl enol ethers are generally planar, but two (67 R = Ph and OMe] derived from an imidazolidinone used in amino acid synthesis show... 

Biochemical reactions include several types of decarboxylation reactions as shown in Eqs. (1)-(5), because the final product of aerobic metabolism is carbon dioxide. Amino acids result in amines, pyruvic acid and other a-keto acids form the corresponding aldehydes and carboxylic acids, depending on the cooperating coenzymes. Malonyl-CoA and its derivatives are decarboxylated to acyl-CoA. -Keto carboxylic acids, and their precursors (for example, the corresponding hydroxy acids) also liberate carbon dioxide under mild reaction conditions. 

Goshe M.B., Anderson V.E. Hydroxyl radical-induced hydrogen/ deuterium exchange in amino acid carbon-... 

It should be emphasized that, in the notation generally used to describe this type of reaction, the a and /3 carbon atoms are the reverse of those usually employed in amino acid chemistry. 

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