Dihydroorotase

Dihydroorotase is an enzyme that plays a crucial role in the biosynthesis of pyrimidine nucleotides, which are essential building blocks of DNA and RNA. It catalyzes the conversion of carbamoyl aspartate to dihydroorotate in the fourth step of the de novo pyrimidine biosynthesis pathway. This enzyme is found in all living organisms, from bacteria to humans, highlighting its fundamental importance in cellular processes.

Structure and Function

Dihydroorotase is a homodimeric enzyme, meaning it is composed of two identical subunits. Each subunit consists of two domains: an N-terminal domain and a C-terminal domain. The N-terminal domain contains the active site, where the catalytic reaction takes place, while the C-terminal domain is involved in stabilizing the overall structure of the enzyme.

The catalytic mechanism of dihydroorotase involves the binding of carbamoyl aspartate to the active site. This substrate undergoes a series of chemical reactions, resulting in the formation of dihydroorotate. The enzyme utilizes a metal ion, typically zinc or iron, as a cofactor to facilitate these reactions.

Role in Pyrimidine Biosynthesis

Pyrimidine nucleotides are essential for DNA and RNA synthesis, as well as other cellular processes such as energy metabolism and signal transduction. The de novo pyrimidine biosynthesis pathway is responsible for the production of these nucleotides from simple precursors.

Dihydroorotase is a key enzyme in this pathway, as it converts carbamoyl aspartate, which is derived from carbamoyl phosphate and aspartate, into dihydroorotate. Dihydroorotate is subsequently converted into orotate by the enzyme dihydroorotate dehydrogenase, leading to the synthesis of uridine monophosphate (UMP), a precursor for all pyrimidine nucleotides.

Clinical Significance

Given its crucial role in pyrimidine biosynthesis, any dysfunction or deficiency in dihydroorotase can have significant consequences. In humans, defects in this enzyme have been associated with rare genetic disorders known as dihydroorotase deficiency and hereditary orotic aciduria.

Dihydroorotase deficiency is characterized by impaired pyrimidine biosynthesis, leading to a decrease in the production of pyrimidine nucleotides. This can result in severe developmental abnormalities, intellectual disability, and growth retardation. Hereditary orotic aciduria, on the other hand, is characterized by the accumulation of orotic acid, a precursor of pyrimidine nucleotides, in the urine. This condition can lead to anemia, failure to thrive, and developmental delays.

See Also

• Pyrimidine Biosynthesis
• Dihydroorotate Dehydrogenase
• Carbamoyl Phosphate Synthetase
• Aspartate Transcarbamoylase