The first step in the catabolism of amino acids is the removal of the nitrogen as ammonia, forming a keto acid that can enter one of the carbon catabolic pathways. the alpha-keto acid pyruvate can be formed from the amino acids alanine, cysteine, glycine, serine and threonine. consider the route for alanine catabolism. anine reacts with to produce pyruvate and. this reaction is catalyzed by aspartate aminotransferase. alanine aminotransferase. alanine dehydrogenase. the substrate for the first step can be regenerated by reacting with nad^+ this reaction is catalyzed by glutamate dehydrogenase. alpha-ketoglutarate dehydrogenase. alanine dehydrogenase. aspartate dehydrogenase. the coenzyme/prosthetic group required in the first reaction is thiamine pyrophosphate. biotin. pyridoxal phosphate. lipoic acid.