BIOINORGANIC CHEMISTRY A Short Course Second Edition

PROTEINS 35

Figure 2.7 A peptide of four amino acids.

NH CH C

CH 2

OH

O

SH

H 2 NCH C

CH 3

O

HN CH C

HC 2 HC 2

O

CH 2

CH 2

NH

C

NH 2

NH

HN CH C

O

HN CH C

CH 2

O

NH

N

Peptide bonds

N-terminal residue C-terminal residue

Direction of peptide chain growth

alanylarginylphenylalanylhistidylcysteine

Peptide Nomenclature

Figure 2.8 Illustration of C α – N bond angle φ (phi) and C α – C bond angle ψ (psi) in a
peptide.

C

C

O

N

C

H

R H

C

O

N

H

C

C

O

H R R

H

N

H

ψ

φ

α C-terminal end

N-terminal end

φ φ

ψ α ψ

α

information and graphic illustrations on proteins and peptides. Two examples
are at http://www.cem.msu.edu/~reusch/VirtTxtJml/protein2.htm#aacd7b and
http://www.moleculesinmotion.com/protarch/page_sheet/menu.html (requires
Chime software; see Chapter 4 ).
The β - pleated sheet structure occurs in fi brous as well as globular proteins
and is formed by intermolecular hydrogen bonds between a carboxyl group
oxygen of one amino acid and an amine hydrogen of an adjacent polypeptide
chain. Parallel β - pleated sheets form when the adjacent polypeptide chains are
oriented in one direction (from N - terminal to C - terminal end or vice versa).
Antiparallelβ - pleated sheets form when the adjacent polypeptide chains are
traveling in opposing directions (one chain N - terminal to C - terminal and the
other C - terminal to N - terminal). Representations are shown in Figure 2.9. The
secondary structure motifs ofα - helix and β - sheet are joined by unstructured
areas called loops or coils, also shown in Figure 2.9.