Tyrosine aromatic nucleus

The aromatic nucleus and one of the methyl groups are derived from homo-gentisic acid, a product of L-tyrosine degradation (D 22). Homogentisic acid reacts with prenylpyrophosphates of different chain length (D 6). The quinols formed are reversibly dehydrogenated to the corresponding quinones, e.g.,... 

Hydroxyl groups which are directly attached to an aromatic nucleus have rather different properties from aliphatic hydroxyl groups since they can act as proton donors and are therefore weakly acidic as in the case of tyrosine (page 37). 

Simple isoquinoline alkaloids may be defined as those containing only one aromatic nucleus and no other cyclic structure except a methylenedioxy substituent. In general, they are derived from only one aromatic amino acid residue e.g. a phenylalanine or tyrosine moiety. 

Dopamine is formed from tyrosine by hyclroxylation with tyrosine hydroxylase and the removal of a CO2 group by aromatic amino acid decarboxylase. The catecholamine is found in high concentrations in parts of the brain—the caudate nucleus, the median eminence, the tuberculum olfactorium, and the nucleus accumbens. Dopamine appears to act as an inhibitory neurotransmitter. 

It has just been shown hat the action of amidases on aromatic derivatives is often accompanied by an oxidation. The following is a new example, though here the deamidization is followed by a fixation on the benzene nucleus of one or more hydroxyl groups. Thus the fonnation of phenol at the expense of the phenylalanin has been observed on diflferent occasions. When phenylalanin is allowed to ferment in the presence of small quantities of peptone, there is found in the Equid oxyphenyl-propionic add, oxyphenylacetic acid, as well as phenol. Now, the quantity of oxyadds formed considerably exceeds in weight that which corresponds to the tyrosin of the peptone the phenylalanin is thus transformed into oxyacids according to- the reactions ... 

Electrophilic substitution of aromatic nuclei in tyrosine and tryptophan side chains has frequently been reported in connection with acidolytic removal of blocking groups. C-Benzylation and tert.butylation of the tyrosine side chain and N-alkylation of the indole nucleus in tryptophan are often attributed to the alkyl cations generated in the reaction. This common side reaction is caused, however, mainly by the alkylating agents formed in the process, such as benzyl bromide or tert.butyl trifluoroacetate. The same is true for the S-alkylation of the methionine side chain. Conversion of the thioether to a sulfonium salt can... 

Of the three aromatic amino acids phenylalanine, tyrosine and tryptophan, phenylalanine has been classified with the hydrophobic amino acids which have a non-polar side chain and tyrosine with those containing a hydroxyl group. This leaves tryptophan, the largest and rarest of the amino acids which contains the heterocyclic indole nucleus as its bulky R group. Trytophan is the parent compound for the neurotransmitter serotonin which is 5-hydroxytryptamine. 

---