ATP

Adenosine triphosphate (ATP) is the primary energy source involved in most of the metabolism. An ATP molecule consists of three parts: a nitrogenous base called adenine, a ribose sugar and three phosphate groups. With the presence of three negatively-charged phosphate groups in ATP, the molecule is highly negative. The pentose sugar in ATP is a ribose, so hydroxyl groups are present at the 2’ and 3’ carbons. N-9 of adenine is linked to 1’ end of ribose through N-glycosidic bond while 𝛼-phosphate group is linked to 5’ carbon. Two additional phosphate groups are connected to 𝛼-phosphate group. Chemical energy is stored in the high energy phosphoanhyride bond between phosphate groups, therefore hydrolysis of ATP into ADP and inorganic phosphate is a highly exogonic reaction. Terminal γ-phosphate is released in hydrolysis. The formation of ATP occurs in anabolic reactions (e.g. photosynthesis) whereas hydrolysis of ATP to release enengy is involved in catabolic reactions (e.g. glycolysis, citric acid cycle and β-oxidation of fatty acids)

GTP

Guanosine triphosphate (GTP) is a purine nucleoside triphosphate. A guanine molecule, a ribose sugar and three phosphate groups are the constituents of GTP. N-glycosidic bond connects N-9 of guanine and 1’ end of ribose whereas 𝛼-phosphate group is linked to 5’ carbon. Similar to ATP, GTP is also involved in various biochemical reactions. GTP is used as an energy source. In citric acid cycle, GTP is produced when succinyl-CoA synthetase converts succinyl-CoA to succinate. Elongation stage of translation also needs the use of GTP. One GTP is used to aid the binding of amino-tRNA onto A site of ribosome. Hydrolysis of another GTP release energy that helps move the ribosome along mRNA. GTP is also involved in signal transduction. The G𝛼 subunit of G protein is activated when it is bound with GTP and it is deactivated when GTP is hydrolysed into GDP and Pi by GTPase.

UTP

Uridine triphosphate (UTP) is a pyrimidine nucleoside triphosphate. UTP consists of a nitrogenous base called uridine, a ribose sugar and three phosphate groups. The 1’ and 5’ carbons of ribose are bonded to N-1 of uracil and 𝛼-phosphate group respectively. N-glycosidic bond is present to link uridine and ribose. UTP is crucial in some metabolic reactions in which it acts as energy source and substrate activator. Its functions are similar to ATP but more specific. In glycogen biosynthesis, glucose-1-phosphate and UTP are converted to UDP-glucose and PPi group by UDP-glucose pyrophosphorylase. Glucose activated by UDP will join the non-reducing end of the growing glycogen chain through α-1⟶4 linkage.