Biochemistry. One of the key substances in the process of biological energy transfer. GTP is a nucleotide, consisting of the purine base guanine, the sugar ribose and three phosphate groups. GTP is involved in energy metabolism, being produced from GDP in the citric acid cycle and serving as energy donator in protein biosynthesis. It is also an activated precursor in the synthesis of ribonucleic acid. GTP is essential to signal transduction, particularly with G proteins.