Catabolism Of Individual Amino Acids 

In this section we shall learn about the catabolism of all twenty amino acids the previous unit we have covered the fate of ammonia release by transdeamination or other means. you know that most amino acids transfer their amino group to a - ketoglutarate while those which utilize alternate ways will be described in the degradation of specific amino acids the common theme is a separate handling of amino nitrogen and carbon skeleton.

the most verstile and important coenzyme in amino acids metabolism is pyridoxal phosphate (PLP) it is best to pick up specific examples of PLP requiring enzyme to appreciate the variety of reactions that are depending on it. the daily requirement of the coenzyme is increased when amino acids catabolism is favored. you will also be introduced to the cofacter such as tetrahydrobiopterin probably for the first time.

the pathways of amino acids catabolism generally contribute only 10% to 15%of the body 's production. the flux through these pathways varies considered as they are reused for biosynthesis or diverted to other pathways ,for instance gluconeogenesis. in the subsection below the catabolism of amino acids is described by grouping them according to the common degradation product  produced.

Amino Acids That Yields Oxaloacetate C4 Family

the amino acids asparagine and aspartic acid produce oxaloacetate. first amide nitrogen of asparagine is hydrolyzed to aspartate by asparaginase this reaction liberates ammonia as a by product. next aspartate transaminase transfer the amino acids group of aspartate to a- ketoglutarate generating oxaloacetate and glutamic acids oxaloacetate may either be converted to glucose or oxidized via the TCA cycle.

Amino Acids That Form a-ketoglutarate (C5 Family)

There are five amino acids that are converted to a-ketoglutarate. they are Gin, Glu, Pro, His and Arg. the amide nitrogen of glutamine like Asn is hydrolyzed by mitochondria glutaminase to ammonia and glutamine. then glutamate dehydrogenase (GDH)the enzyme generally works in the direction of oxidative deamination unless the concentration of ammonia is high as in hyperammonemia the C5 compound ,a-ketoglutarate is gluconeogenic and can also replenish TCA cycle intermediate or is completely oxidized.

the amino acids proline ,arginine and histidine are also converted to glutamate and then deaminated. proline is converted to glutamate in three steps ,one of which is non enzymatic. it is oxidized to pyrroline 5-carboxylate which spontaneously hydrolyses on addition of water to glutamate y-semialdehyde which on oxidation yields glutamate.

arginine is catabolized in mammals through the urea cycle. it is converted into ornithine and urea by arginase. you have studied this enzyme and its distribution in the last unit. next ornithine is transaminated by ornithine amino transferase to glutamate y-semialdehyde. the ketoacid in this reaction is a -ketoglutarate which is converted to glutamate. glutamate y-semialdehyde also comes from proline catabolism ,thereby linking the two pathways. it is oxidation to glutamate by NAD+ -dehydrogenase.