Bone and Cartilage – its Structure and Physical Properties - Biomechanics of Hard Tissues - page 47

R ″

NH CH CO NH CH CO NH CH CO

R ′ R ′′′

Figure 1.30 ''Amino acids in chains are the cause, so the
X-ray explains, of the stretching of wool and its strength
when you pull, and show why it shrinks when it rains''
(A. L. Patterson). After Astbury [200].

Figure 1.31 An α -amino acid molecule, with the amino
group on the left and the carboxyl group on the right.
After Pauling Chemistry [9].

H

H

H

O

NCC

H O

R

carboxyl groups are attached to the same carbon, which is called the α -carbon .The

various α -amino acids differ in the side chain (R group) that is attached to their

α -carbon.

Twenty standard amino acids are used by cells in protein biosynthesis, and these
are specified by the general genetic code. These 20 amino acids are biosynthesized
from other molecules. Organisms differ in the types of amino acids synthesized
by them and the ones that are obtained from food. The amino acids that cannot
be synthesized by an organism are called essential amino acids .Ofthe20standard
amino acids, 8 are essential amino acids.
Glycine and proline, used in building the collagen chain, belong to nonessential
amino acids. Hydroxyproline, also appearing in the collagen, is a modification of
proline: hydroxyproline differs from proline by the presence of additional hydroxyl
(OH) group.
Glycine (abbreviated as Gly or G) is the organic compound with the formula
NH 2 CH 2 COOH and is considered a glucogenic amino acid; it is the smallest of the
20 amino acids. Most proteins contain only small quantities of glycine. An exception
is collagen, which contains almost one-third of glycine. Glycine is a colorless,
sweet-tasting crystalline solid. It is unique among the proteinogenic amino acids in
that it is not chiral. It can fit into both hydrophilic and hydrophobic environments,
because of its single hydrogen atom side chain. Glycine was discovered in 1820 by
Braconnot, who obtained a gelatin sugar , named later as glycocolle. It is now called
glycine [201].
Proline (abbreviated as Pro or P) is an -amino acid, one of the 20 DNA-encoded

amino acids. Its molecular formula is C 5 H 9 NO 2 . The distinctive cyclic structure
of the proline side chain gives proline an exceptional conformational rigidity
as compared to other amino acids. Hydroxyproline (abbreviated as Hyp )isan
uncommon amino acid and differs from proline by the presence of a hydroxyl (OH)
group attached to the C atom.

α