glutathione synthetase to form glutathione. in chemistry, peptides are synthesized by a variety of reactions. one of the most-used in solid-phase peptide synthesis uses the aromatic oxime derivatives of amino acids as activated units. these are added in sequence onto the growing peptide chain, which is attached to a solid resin support. libraries of peptides are used in drug discovery through high-throughput screening. the combination of functional groups allow amino acids to be effective polydentate ligands for metal–amino acid chelates. the multiple side chains of amino acids can also undergo chemical reactions. catabolism catabolism of proteinogenic amino acids. amino acids can be classified according to the properties of their main degradation products: * glucogenic, with the products having the ability to form glucose by gluconeogenesis * ketogenic, with the products not having the ability to form glucose. these products may still be used for ketogenesis or lipid synthesis. * amino acids catabolized into both glucogenic and ketogenic products. degradation of an amino acid often involves deamination by moving its amino group to alpha-ketoglutarate, forming glutamate. this process involves transaminases, often the same as those used in amination during synthesis. in many vertebrates, the amino group is then removed through the urea cycle and is excreted in the form of urea. however, amino acid degradation can produce uric acid or ammonia instead. for example, serine dehydratase converts serine to pyruvate and ammonia. after removal of one or more amino groups, the remainder of the molecule can sometimes be used to synthesize new amino acids, or it can be used for energy by entering glycolysis or the citric acid cycle, as detailed in image at right. complexation amino acids are bidentate ligands, forming transition metal amino acid complexes. chemical analysis the total nitrogen content of organic matter is mainly formed by the amino groups in proteins. the total kjeldahl nitrogen (tkn) is a measure of nitrogen widely used in the analysis of (waste) water, soil, food, feed and organic matter in general. as the name suggests, the kjeldahl method is applied. more sensitive methods are available.