PROTEINS SKIN, LEATHER, AND GLUE - Archaeological Chemistry - page 326

to totally break down the shape of the proteins. Proteins that lose their
shape because of an increase in temperature are said to be denaturated .
Soft- and hard-cooked eggs and cooked meat are examples of denaturated
proteins.
Enzymes
The enzymes are complex proteins that catalyze reactions (alter the rate
at which chemical reactions proceed) in living organisms, but are not them-
selves altered. The synthesis of the enzymes, as that of all the proteins in
the living body, is regulated by DNA when proteins are synthesized.
Enzymes are active, for example, in the formation of biopolymers in the
body of living organisms, as when amino acids polymerize into proteins.
The synthesis of proteins, which without enzymes might take extremely
long periods of time to complete, takes very short times, a matter of
seconds or even a fraction of a second when catalyzed by enzymes.
Enzymes also accelerate the synthesis of DNA and RNA. Enzymes known
by the generic name polymerases are used in the polymerase chain reac-
tion (PCR) for enlarging fragments of DNA (see Textbox 64).
Most enzymes are very specific in their activity, and each chemical reac-
tion in a living organism requires a specific enzyme. Their specificity arises
from what is known as an active site , a location in the enzyme's molecule
that has a shape matching that of a part of the molecule with which it
reacts. The activity of the enzymes is affected by such factors as tempera-
ture and pH, each enzyme functioning best within a specific range of tem-
peratures and pH. Outside this range the enzymes are structurally altered
and their activity is either impaired or terminated.
The proteins are complex substances, much more complex than the car-
bohydrates or lipids, the other main components of the body and the sources
of energy of living organisms. When carbohydrates or lipids are in short
supply, living organisms can replace them by proteins that are then chemi-
cally altered and made to release energy. There are, however, no substances
that can take the place of proteins in the building of new cells, the mainte-
nance of existing ones, or the replacement of dead cells in the body
(Creighton 1983). One of the most essential proteins in animal tissues, par-
ticularly vertebrates, is collagen , the main component of skin and a major
component of bone, cartilage, and connective tissues (see Textbox 61).