Page 40 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 25

of consumption for maintenance of (A) COOH
VetBooks.ir hydration can be severely compromised
if an animal is not able to absorb the
water correctly from its intestines;
severe dehydration can occur in animals Leucine
with signs of vomiting or diarrhea.
H 2 N
Proteins
Lysine Arginine
After water, proteins are the next largest
constituent of protoplasm. Proteins are
complex high‐molecular‐weight colloidal Histidine Phenylalanine
molecules consisting primarily of amino
acids that are polymerized (joined) into
polypeptide chains (Fig. 2‐2A). Twenty
different amino acids are specified by the (B)
mammalian genetic code and are combined
to form a variety of proteins. The union of
amino acids within a protein molecule is
by way of a peptide linkage, a bond between
the amino (NH ) group of one amino acid Chemical
bonds
2
and the carboxyl (COOH) group of another
amino acid, with the elimination of water.
A small chain of amino acids is called a
peptide. A polypeptide is a chain of more
than 50 amino acids connected by peptide
linkages, and a chain that contains more
than 100 amino acids is called a protein.
These chains of amino acids are referred to
as the primary structure of the protein.
The peptide linkages between amino Figure 2-2. (A) A chain of amino acids joined
acids in a protein are somewhat flexible, and by peptide bonds to form a protein. (B) A large
this permits the chain to bend into various protein. Each filled circle represents a single
three‐dimensional shapes (Fig. 2‐2B). The amino acid. Chemical bonds between amino acids
at distant points in the chain produce the three‐
configurations in this secondary structure dimensional shape of the protein molecule.
may become relatively stable, because
chemical attractions (hydrogen bonds)
form between amino acids at various points proteins, and examples of each, are as
in the chain. The final three‐dimensional follows:
shape, or tertiary structure, of a protein is 1. Albumins (plasma albumin, milk
an important determinant of its biologic lactalbumin)
function, because the shape can determine 2. Globulins (plasma globulins, globulins
what segments of the protein chain are in plant seeds)
exposed and available to interact with other 3. Protamines (in sperm cells)
molecules. 4. Histones (with nucleoproteins in cell
Amino acids, and thus proteins, contain
nuclei)
carbon, hydrogen, oxygen, and nitrogen. 5. Albuminoids (collagen and elastin of
Proteins may also contain other elements connective tissue)
such as sulfur, phosphorus, or iron. Simple
proteins yield only amino acids or their Conjugated proteins consist of simple
derivatives upon hydrolysis. The simple proteins combined with a component that