Thyroid hormones, melanin

In addition to their well known role in protein structure, amino acids also act as precursors to a number of other important biological molecules. For example, the synthesis of haem (see also Section 5.3.1), which occurs in, among other tissues, the liver begins with glycine and succinyl-CoA. The amino acid tyrosine which maybe produced in the liver from metabolism of phenylalanine is the precursor of thyroid hormones, melanin, adrenaline (epinephrine), noradrenaline (norepinephrine) and dopamine. The biosynthesis of some of these signalling molecules is described in Section 4.4. 

In general, there are protein kinases and, in particular, there are tyrosine kinases tyrosine is a nonessential amino acid present in most proteins and is synthesized metabolically from the essential amino acid phenylalanine. It is also said to be a precursor of thyroid hormones, melanin, and catecholamines. Not only is there more than one tyrosine kinase, but mutations can also occur. 

Tyrosine is a (crystalline) amino add with the formula HOQH4CH2CH(NH2) COOH. It results from the hydrolysis of proteins, that is, the reaction of proteins with water. The -C6H4- group within the compoimd denotes a ring-type stmcture. Another definition provided is that tyrosine is found in most proteins and can be synthesized metabolically from phenylalanine. Furthermore, tyrosine is a precursor for thyroid hormones, melanin (the dark pigment of body parts, notably, the skin), and catecholamines (that affect the sympathetic nervous system). 

Tyrosine is a precursor of thyroid hormones as well as L-dopa. Both thyroxine and L-dopa are employed in clinical medicine thyroxine to treat hypothyroid patients and L-dopa to treat patients with Parkinsonism. L-dopa is also the precursor to the pigment of the skin known as melanin. The enzyme that catalyzes the transformation of tyrosine into L-dopa, tyrosine hydroxylase, also catalyzes the transformation of L-dopa into melanin. Albinism is a genetic disease in which a mutation in the gene encoding tyrosine hydroxylase results in an inactive enzyme. People with albinism have no pigment in their skin, hair, or retina. 

Thyroid hormone, 3,4-dihydroxy phenylalanine (dopa), melanin, dopamine, norepinephrine, and epinephrine are produced from tyrosine. 

Insulin Melanin-concentrating hormone Thyroid hormones... 

Phenylalanine and tyrosine are also metabolized in higher organisms by two routes which are quantitatively less important but physiologically of the highest importance. The first leads to the adrenal hormones adrenaline (epinephrine) and noradrenaline (norepinephrine),which may be formed as in diagram 11 this pathway also leads to melanin (diagram 12). The second leads to the thyroid hormones thyroxine and triiodothyronine, the synthesis and breakdown of which are also discussed. 

The answer is e. (Murray, pp 307-346. Scriver, pp 1667—1724. Sack, pp 121-138. Wilson, pp 287—3177) In humans, tyrosine can be formed by the hydroxylation of phenylalanine. This reaction is catalyzed by the enzyme phenylalanine hydroxylase. A deficiency of phenylalanine hydroxylase results in the disease called phenylketonuria [PKU(261600)]. In this disease it is usually the accumulation of phenylalanine and its metabolites rather than the lack of tyrosine that is the cause of the severe mental retardation ultimately seen. Once formed, tyrosine is the precursor of many important signal molecules. Catalyzed by tyrosine hydroxylase, tyrosine is hydroxylated to form L-dihydroxyphenylalanine (dopa), which in turn is decarboxylated to form dopamine in the presence of dopa decarboxylase. Then, norepinephrine and finally epinephrine are formed from dopamine. All of these are signal molecules to some degree. Dopa and inhibitors of dopa decarboxylase are used in the treatment of Parkinson s disease, a neurologic disorder. Norepinephrine is a transmitter at smooth-muscle junctions innervated by sympathetic nerve libers. Epinephrine and dopamine are catecholamine transmitters synthesized in sympathetic nerve terminals and in the adrenal gland. Tyrosine is also the precursor of thyroxine, the major thyroid hormone, and melanin, a skin pigment. 

Behaves similarly to phenylalanine and tryptophan (see phenylalanine). Precursor of melanin, epinephrine, and thyroid hormones. 

Tyrosine (Tyr or Y) (4-hydroxyphenylalanine ((5)-2-amino-3-(4-hydroxyphenyl)-propanoic acid)) is a polar, neutral, aromatic amino acid with the formula H00CCH(NH2)CH2C6H50H and is the precursor of thyroxin, dopamine, norepinephrine (noradrenaline), epinephrine (adrenaline), and the pigment melanin. Being the precursor amino acid for the thyroid gland hormone thyroxin, a defect in this may result in hypothyroidism. Tyr is extremely soluble in water, a property that has proven useful in isolating this amino acid from protein hydrolysates. The occurrence of tyrosine- 0-sulfate as a constituent of human urine and fibrinogen has been reported. ... 

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