Series editors' preface - Wood Tools

(c)
4.7.4 Proteins
Plastics and polymers, coatings and binding media, adhesives and consolidants 169
R 1 H O R 3 H O R 5
CH N C CH N C CH
N C CH N C CH N C
H O R 2 H O R 4 H O
peptide backbone
R side chain groups or residues
Proteins are naturally occurring polymers consisting
of large numbers of repeating -amino
acid units. The general formula of an amino
acid is shown in Figure 4.13a. Each amino acid
has its own side chain groups or residues (R)
(Figure 4.13b). The term peptide bond refers
to the amide group (⎯CO⎯NH⎯) that joins
two -amino acid units together (Figure 4.13c).
The type and sequence of -amino acid side
chain groups gives each polypeptide unique
properties.
All -amino acids contain both acidic
(⎯COOH) and basic (⎯NH 2⎯) groups and
are therefore amphoteric (i.e. capable of reacting
with both acids and bases). More complex
amino acids may contain two carboxyl groups
(acidic) two amino groups (basic), aromatic or
heterocyclic ring structures or may also contain
sulphur.
Proteins, which may have molecular weights
from a few thousand to several million, are
divided into two general groups, the fibrous
(structural) proteins and the globular (regulatory)
proteins. Fibrous proteins such as keratin
and collagen consist of long thread-like chains
joined laterally by various types of crosslinkages
to form fairly stable and insoluble structures.
Biologically active proteins such as
enzymes are of the globular type in which a
considerable amount of folding of the long
polypeptide chains occurs to give a globular or
somewhat elliptical shape overall. Four structural
levels are assigned to describe the complex
structure of proteins. The primary
structure simply describes the linear sequence
of amino acids in a polypeptide chain (Figure
4.14a). Interaction of regularly occurring
groups on the polypeptide chain yields a secondary
structure (Figure 4.14b). There are several
types of secondary structure, of which two
common examples are the -helix and the -
(c) A section of a polypeptide
chain showing the peptide
backbone and side chain groups
or residues (R)
sheet. The -helix structure has a chain of
repeating amino acid units wound into a spiral
which is stabilized by hydrogen bonds
between carbonyl (CO) and imido (NH)
groups which occur at regular intervals along
the chain. In the -sheet configuration, which
occurs for example in silk, two or more peptide
chains are held together laterally by hydrogen
bonds into an orderly crystalline structure.
The tertiary structure (Figure 4.14c) describes
the overall three-dimensional structure (i.e. the
coiling and folding) of a globular protein molecule.
The quaternary structure describes the
spatial arrangement of a protein characterized
by a number of sub-units of identical tertiary
structure (Figure 4.14d). This commonly
occurs with enzymes.
The term ‘denatured’ describes the partial or
complete disruption of the arrangement of
polypeptide chains within a protein. Once
denatured, the protein may become insoluble
(e.g. a boiled egg) or lose its function (e.g. an
enzyme). Changes in pH, temperature, salt
concentration and the presence of reducing
agents can denature proteins. Some solvents
can also denature proteins.
Collagen
By far the most common adhesive for use in
furniture was animal protein glue. Before the
twentieth century, the unqualified term ‘glue’
meant this and only this owing to the widespread
use of animal glue for most purposes.
Animal glues have great importance in the history
of both woodworking and conservation
and are discussed in detail by Grant (1980) and
Ward (1977). The primary component of animal
glues and sizes is gelatin derived from collagen
(Greek: colla, ‘glue’; gen, ‘creator’), the
long fibrous structural protein of connective
tissue present in animal skins, muscle, bone
and hide. Although collagen itself is insoluble
in water, glue can be made from it by