Ribonucleotide, biosynthesis catabolism

NAD tends to be an electron acceptor in catabolic reactions involving the degradation of carbohydrates, fatty acids, ketone bodies, amino acids, and alcohol. NAD is used in energy-producing reactions. NADP, which is cytosolic, tends to be involved in biosynthetic reactions. Reduced NADP is generated by the pentose phosphate pathway (cytosolic) and used by cytosolic pathways, such as fatty acid biosynthesis and cholesterol synthesis, and by ribonucleotide reductase. The niacin coenzymes are used for two-electron transfer reactions. The oxidized form of NAD is NAD". There is a positive charge on the cofactor because the aromatic amino group is a quaternary amine. A quaternary amine participates in four... 

Nucleotide biosynthesis, like nucleotide catabolism, is relatively complex. Thus, we ll again look at only one example, adenosine monophosphate. Purine nucleotides are formed by initial attachment of an -NH2 group to ribose, followed by multistep buildup of the heterocyclic base. The attachment of -NH2 takes place by a nucleophilic suhstitution reaction of ammonia with 5-phosphoribosyl a-diphosphate to give /3-5-phosphorihosylamine and probably involves an SNl-like loss of diphosphate ion with formation of an 0x0-nium-ion intermediate. Although we ll not cover the details of its formation, inosine monophosphate (IMP) is the first fully formed purine ribonucleotide, with adenosine monophosphate (AMP) derived fi-om it. 

In summary, the biochemical function of folate coenzymes is to transfer and use these one-carbon units in a variety of essential reactions (Figure 2), including de novo purine biosynthesis (formylation of glycinamide ribonucleotide and 5-amino-4-imidazole carboxamide ribonucleotide), pyrimidine nucleotide biosynthesis (methylation of deoxyuridylic acid to thy-midylic acid), amino-acid interconversions (the interconversion of serine to glycine, catabolism of histidine to glutamic acid, and conversion of homocysteine to methionine (which also requires vitamin B12)), and the generation and use of formate. 

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