Question

Discus the Biochemistry of the important physiologically active amines derived from tyrosine

Answer 1

Tyrosine is an amino acid. As we know, amino acids are the building blocks of proteins.

Tyrosine is the precursor for the synthesis of a group of amine neurotransmitters. Neurotransmitters help to transfer the information across a synapse (junction between two neurons).

Amine neurotransmitters include three catecholamines - having a catechol moiety; dopamine, norepinephrine, and epinephrine.

- The first step in the pathway is catalyzed by tyrosine hydroxylase. It requires oxygen as a co-substrate. The product formed in DOPA (Dihydroxyphenylalanine). This step is the major rate-limiting in the synthesis of other transmitters.

- The second enzyme, DOPA decarboxylase produces dopamine through the removal of a CO2 molecule (decarboxylation). Dopamine is predominantly concentrated in the area corpus striatum of the brain. Once released, dopamine binds to beta-adrenergic receptors as well as dopamine receptors. It receives input from substantia nigra and plays a key role in the coordination of body movements. In Parkinson's disease, the dopaminergic neurons present in substantia nigra degenerate leading to motor dysfunction.

- The third enzyme, Dopamine beta-hydroxylase catalyzes the synthesis of norepinephrine (noradrenaline) from dopamine, by utilizing oxygen. Dopamine is transported in specific vesicles to the adrenergic terminals, where it gets converted to norepinephrine. Neurons that synthesize norepinephrine are the sympathetic ganglion cells, and it plays a key role in the visceral motor system. Physiologically, norepinephrine works to lower the heart rate. It a common neurotransmitter of the sympathetic nervous system.

- The fourth enzyme, phenylethanolamine N-methyl transferase catalyzes the synthesis of epinephrine from norepinephrine. Epinephrine is present in lower levels in the brain compared to other catecholamines. It is considered the fight or flight hormone. It acts to increase the heart beat and increases blood glucose levels. It is the common neurotransmitter of the parasympathetic nervous system.

Physiologically, epinephrine, and norepinephrine have contrasting effects on the human body.