Amino acid degradation:

As there is no store for excess amino acids, and as proteins are constantly being turned over, amino acids have to be repeatedly degraded.  The α-amino group is erased first and the resulting carbon skeleton is transformed into one or more main metabolic intermediates and used as metabolic fuel. The carbon skeletons of the 20 standard amino acids are funneled into only seven molecules that are as follows: acetoacetyl CoA, pyruvate, succinyl CoA, acetyl CoA, α-ketoglutarate, oxaloacetate and fumarate.  Amino  acids  which  are  degraded  to  pyruvate, succinyl  CoA,  α-ketoglutarate, oxaloacetate  and fumarate are termed glucogenic  as they can produce increse to the net synthesis of glucose. This is just because the citric acid cycle intermediates and pyruvate can be transformed into phosphoenolpyruvate and then into glucose through gluconeogenesis. By comparison amino acids that are degraded to acetyl CoA or acetoacetyl  CoA are termed ketogenic since  they  produce  rise  to  ketone  bodies;  the  acetyl  CoA  or acetoacetyl CoA can also be used to synthesize lipids. Of the standard set of 20 amino acids, only Lys and Leu are solely ketogenic.  Phe, Trp Ile, and Tyr are both ketogenic and glucogenic as some of their carbon atoms end up in  acetyl  CoA  or  acetoacetyl  CoA,  while  others  end  up  in  precursors  of glucose and the remaining 14 amino acids are classified as solely glucogenic.