Norepinephrine biologically occurring

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. 

Ascorbic acid (AA) is an antioxidant, which often coexists with DA under physiological conditions at levels ranging from 200 to 400 pM [199, 200]. The oxidation of AA is irreversible and occurs at potentials close to that of DA and norepinephrine on conventional electrodes, such as Au, Pt, and GC [200-202]. Thus, the development of electrodes that allow the simple and rapid determination of DA with high selectivity and sensitivity is highly desirable for biological diagnostic applications. 

The enzymatic introduction of a functional group into a biologically important molecule is not only specific with regard to the location at which the reaction occurs in the molecule (see Chapter 4, Problem 50), but also usually specific in the stereochemistry obtained. The biosynthesis of epinephrine first requires that a hydroxy group be introduced specifically to produce (—(-norepinephrine from the achiral substrate dopamine. (The completion of the synthesis of epinephrine wdl be presented in Problem 71 of Chapter 9.) Only the (—) enantiomer is functional in the appropriate physiological manner, so the synthesis must be highly stereoselective. 

The naturally occurring catecholamines—dopamine (DA) (1), / -norepinephrine (/ -NE) (2), and / -epinephrine (/J-EPI) (3)— have many imiK)rtant biological functions. These catecholamines are produced in vivo from L-tyrosine. Tyrosine is first converted to dihydroxyphenylalanine (DOPA) by aromatic hydroxylation. L-DOPA is then decarboxylated to give DA, which is subsequently converted to / -NE by p-hydroxylation. DA is a vital neurotransmitter in the central nervous system (CNS) and has actions on the kidneys and heart. Norepinephrine is also present as a neurotransmitter in the CNS, and is the principal neurotransmitter of the peripheral sympathetic nervous system. Epinephrine, which is elaborated from / -NE by N-methylation in the adrenal medulla, has potent actions on the heart, smooth muscle, and other organs (/). 

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