Pharmacology for Anaesthesia and Intensive Care

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Section IBasic principles

Building blocks: amino acids, nucleic acids and sugars

Amino acids

The basic structure of anα-amino acid is a hydrocarbon group with both a carboxyl

and amine group attached to the end carbon (theα-carbon). There are 20 com-

monly occurringα-amino acids that form the building blocks for protein synthesis

(of which 5 cannot be synthesized – the essential amino acids). Not all amino acids

form peptides and proteins; some amino acids are important precursors in neuro-

transmitter synthesis. For example phenylalanine can be metabolized to tyrosine

which then enters adrenergic neurones as the substrate for catecholamine synthesis

(see Chapter 12 ). Otherα-amino acids are central neurotransmitters in their own

right, for example, glycine and glutamate. Not all amino acids of importance are

α-amino acids. GABA (γ-amino butyric acid), as its name suggests, is aγ-amino acid

that has the carboxyl and amine groups on opposite ends of a butyl backbone; GABA

is an important inhibitory neurotransmitter.

RNH 2 CH 2

CH 2

COOH

CH 2 C NH 2

H

H

CH

NH 2

α

α βγ

βα

C

H

COOH

An α-amino acid

GABA – γ-amino butyric acid

Phenylalanine Glycine

HNH 2

α

C

H

COOH COOH

Nucleic acids, nucleosides and nucleotides

Nucleosides are formed from the combination of a nucleic acid with a sugar, usually

ribose (e.g. adenosine, guanosine). Nucleotides are the building blocks of DNA/RNA

and are formed from nucleosides linked to a phosphate group. The nucleic acids

are either purines (adenine or guanine) or pyrimidines (cytosine, uracil or thymine).

Many anti-cancer drugs are analogues of nucleic acids or nucleotides. Nucleosides

are important intermediates in metabolic processes as they can combine with high-

energy phosphate groups to act as co-factors in metabolic and catabolic processes

within the cell.