Ornithine serves three metabolically important roles, but is not found in proteins. what is one of the important roles of ornithine?
a. amine donor for many transamination reactions
b. an intermediate in the urea cycle
c. involved in the synthesis of serine, which serves as a precursor for glycine and cysteine
d. its reaction with methenyl tetrahydrofolate produces lysine
e. none of the above

b. an intermediate in the urea cycle

Explanation:

Ornithine is a non-essential amino acid which is produced as intermediate molecule in urea cycle.

Ornithine also plays important role in regulation of several metabolic processes which leads to diseases like hyperammonemia, hyperorithinemia and cancer in the body of humans.

Role in the urea cycle:

Ornithine is a products of action of enzyme arginase on l-arginine which is responsible for the formation of urea. Thus, ornithine is a central part of the urea cycle and allows for disposal of the excess amount of nitrogen.