Creatine is present in the tissues (muscle, brain, blood, and so on) as the high energy compound, phosphocreatine, and as free creatine. Three amino acids – glycine, arginine, and methionine – are required for the formation of creatine. The first reaction occurs in kidney. It involves the transfer of guanidine group of arginine to glycine,…

Thyroid hormones, thyroxine and triiodothyronine, are synthesised from the tyrosine residues of the protein thyroglobulin and activated iodine. Iodination of tyrosine ring occurs to produce mono-iodination and diiodotyrosine, from which triiodothyronine (T3) and thyroxine undergo proteolytic breakdown to release the free hormones, T3 and T4.

Phenylalanine is converted into tyrosine, which is further converted into DOPA by tyrosine hydroxylase with tetrahydropteridine as cofactor. DOPA is decarboxylated to dopamine by a decarboxylase, which is present in many tissues, including a drenal medulla with pyridoxal phosphate as cofactor. The hydroxylation of dopamine is carried out by dopamine β-hydroxylase to form norepinephrine in…

Melanin is the pigment of skin, hair, and eye. The synthesis of melanin occurs in melanosomes present in melanocytes, the pigment-producing cells. Tyrosine is the precursor for melanin and only one enzyme, namely tyrosinase, is involved in its formation. Tyrosinase hydroxylates tyrosine to form 3,4-dihydroxyphenylalanine (DOPA). DOPA can act as a cofactor for tyrosinase. The…

As phenylalanine is converted to tyrosine, a single pathway is responsible for the degradation of both these amino acids, which occurs mostly in liver. In a sequence of reactions, tyrosine is converted to fumarate and acetoacetate. Tyrosine first undergoes transamination to 5-p-hydroxyphenyl pyruvate. P-hydroxyphenyl pyruvate dioxygenase is a copper-containing enzyme. It catalyses oxidative decarboxylation as…

Phenylalanine is hydroxylated at para position by phenylalanine hydroxylase to produce tyrosine. This is an irreversible reaction and requires the participation of a specific coenzyme biopterin. The active forms of biopterin is tetrahydrobiopterin, in the reaction it is converted to dihydrobiopterin. In the phenylalanine hydroxylase reaction, tetrahydrobiopterin. Tetrahydrobiopterin is then regenerated by an NADPH-dependent dihydrobiopterin…

Urea cycle is linked with TCA cycle in three different ways:

The first reaction catalysed by CPS I is the rate-limiting enzyme reaction or committed step in urea synthesis. CPS I is allosterically activated by NAG. It is synthesised from glutamate and acetyl CoA by synthase and degraded by a hydrolase. The rate of urea synthesis in liver is correlated with the concentration of NAG. The…

The skeleton of urea consists of two amino (—NH2) groups and one carbon atom. The first (NH2) group is derived from glutamate reservoir (—NH2) and the second (—NH2) group is derived from aspartate. The carbon atom is derived from CO2. The urea cycle occurs in five steps with five enzymes. The first two enzymes are…

Urea is processed in two locations; the synthesis occurs in liver and it is transported to kidney for excretion. Urea cycle was first described by Hans Krebs and Kurt Henseleit in 1932. Hence, this cycle is also known as Krebs–Henseleit cycle.