Introduction to Human Nutrition

52 Introduction to Human Nutrition

H
|
subsequent fi gures represent the – C – COOH moiety).
|
NH 2

The carboxylic acid and amino nitrogen groups are
the components of the peptide bond that links the

amino acids within the linear peptide structure, while

the side-chains distinguish the physical and chemical

properties of each chemical class of amino acid. In

addition, some features of the amino acid side-chains

are critical to the metabolic and physiological roles of

free, as opposed to protein-bound, amino acids (Table

4.3; Figures 4.1 and 4.2). These roles are refl ections of

(c)

(a) (b)

H

HO CH 2

CH 2 CH^2

CH 2

COOH

N

H

A

A

A

A

N N

H N

H

Tyrosine Proline

Histidine

Phenylalanine

Tryptophan

Figure 4.1 Structures of some of the nutritionally important amino acids. All are components of proteins and they are coded by DNA.
(a) Nutritionally indispensable (essential) includes also tryptophan and histidine; (b) nutritionally conditionally indispensable; (c) nutritionally
dispensable.