Classification Of amino Acids Based On The Fate Of The Carbon Skeleton

Amino acids are classified as either glycogenic ,ketogenic or both. Amino acids whose carbon skeleton can be converted to glucose are glycogenic whereas those which can be converted to ketone bodies or fatty acids are ketogenic. a few amino acids are both glycogenic and ketogenic as their partial degradation generation two product, one of which is glycogenic and the other ketogenic.

This classification does not hold true for organism that possess the glyoxylate cycle. in that case there is strictly no ketogenic amino acids since they can convert acetylCoA to glucose. even in animals tissues, particularly liver ,the enzymes of glyoxylate cycle are expressed to some extent. yet we still continue with this classification inspite of its limitation. the division of amino based on the above criteria is given in 

Convergent pathway of amino acid catabolism 

A small percentage of amino acids released from protein are catabolism. it generally begins with a different set of enzyme for each amino but some amino acids do have few shared steps ,as in case of branch chain amino acids the degradation of amino acids yields intermediate which are either directly fed to the krebs cycle or after few modification. you would recall from carbohydrate metabolism that krebs cycle is the convergent cycle for the complete oxidation of carbon emerging from all classes of biomolecules.

The twenty amino acids are grouped according to the common degradation product generating these intermediate are oxaloacetate, pyruvate ,a ketoglutarate ,succinyl coa fumarate, acetoacetate  and acetylcoa the classification is ambiguous as some amino acids like threonine are degraded by more than one pathway which may not always end with the same product. in addition there are amino acids (phenylalanine and tyrosine )which produce more than one product and therefore and therefore can be placed in more than one group. the entry of amino acids into the TCA cycle 

condition favoring catabolism of amino acids

in general carbohydrate have a sparing effect on protein catabolism. the release of amino acids from protein turnover varies considerably depending on the situation. some of these metabolism condition are as follow:

during normal turnover of cellular protein ,only a fraction of amino acids are degraded.

consumption of protein rich diet enhances the rate of degradation as excess amino acids are not stored.

during starvation and uncontrolled diabetes mellitus(DM),when carbohydrate are unavailable or improperly utilized, cellular proteins are used as fuel.