The nitrogenous part is converted into urea for excretion while the carbon skeletons may be used in various ways. In principle, the carbon skeletons are ultimately oxidized to provide energy.
Some amino acids are both ketogenic and glucogenic.
Thus it is said that some amino acids are essential and some are nonessential . When proteins are being synthesized, an adequate supply of all 20 amino acids must be present. The system cannot 'wait' until an amino acid turns up and nor can it continue in the absence of one of the amino acids. The body therefore needs to operate its amino acid economy very carefully. In the absence of a storage mechanism, it must bring together all 20 amino acids in the cell cytosol for protein biosynthesis, synthesizing those it can, if necessary, and obtaining the others in the diet.