Dicarboxylic acids, synthesis from carbohydrates

The reaction of type i is utilized for the preparation of a variety of dicarboxylic acids. Scheme 25 shows the synthesis of a C-disaccharide (73) from a carbohydrate carboxylic acid (72) [98]. 

The steps of the citrate cycle involving dicarboxylic acids— from succinate to oxaloacetate—are even richer in correlations with other metabolic pathways. Fumarate is a fragment of the breakdown of tyrosine it is also formed from aspartate in the course of the formation of urea (see below). Oxaloacetate can be converted by reversible transamination into aspartate, one of the nonessential amino acids. Another pathway leads from oxaloacetate to phosphoenolpyruvate and hence to the synthesis of carbohydrates. 

Carbon Metabolism of Amino Acids. Nine of the 18 common amino acids are related directly or indirectly to the dicarboxylic acids of the citrate cycle. Proline, histidine, and arginine (or ornithine) produce glutamate and thence a-ketoglu-tarate (on the chart, below the citrate cycle). The aromatic amino acids are broken down to fumarate, which is also formed from aspartate in the urea cycle. Lastly, alanine should be included, since it can enter the citrate cycle via pyruvate and acetyl-CoA. The fact that the carbon chains of amino acids enter the citrate cycle is important both for their complete degradation and for their conversion to carbohydrates (as mentioned already, cf. Chapt. XV-10). The synthesis of amino acids cannot always proceed by a reversal of the breakdown many of them are essential components of the diet (cf. Chapt. XXII-2). 

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