Page 82 - Small Animal Clinical Nutrition 5th Edition

82 Small Animal Clinical Nutrition

configuration of the polypeptide chains resulting from hydro- Function
VetBooks.ir gen bonds between adjacent amino acids. Alpha-helices, β- Proteins are the principal structural constituents of body organs
and tissues including: 1) collagen and elastin found in cartilage,
pleated sheets or random coils are formed by these hydrogen
tendons and ligaments, 2) the contractile proteins actin and
bonds. The tertiary structure describes how further interac-
tions of the amino acids cause folding and bending of the myosin in muscles, 3) keratin proteins in skin, hair and nails
polypeptide chain giving the protein its biologic activity. and 4) blood proteins including hemoglobin, transferrin, albu-
Proteins have a quaternary structure if they contain more than min and globulin. Proteins also function as enzymes, hormones
one polypeptide chain. Hydrogen, electrostatic and ionic (e.g., insulin) and antibodies. Amino acids can serve as a source
bonds form between the polypeptide chains and stabilize the of energy after the nitrogen-containing amino group is
aggregates. The primary structure of proteins is responsible removed by deamination or transamination.
for the secondary, tertiary and quaternary structures that are
formed. Importance of Amino Acids
Other compounds bound to peptides may also classify pro- Several amino acids are classified as essential or indispensable
teins. Simple proteins are made up of only amino acids. Simple (10 for dogs and 11 for cats).These amino acids cannot be syn-
proteins are subclassified into fibrous or globular proteins thesized by the body in sufficient quantities and therefore must
according to shape, solubility and chemical composition. be supplied by food. The carbon skeletons of these essential
Fibrous proteins include collagens, elastins and keratins, which amino acids are the critical component that the body cannot
are the major structural proteins in the body. Collagens are the synthesize. Many of the remaining amino acids are nonessen-
main proteins of connective tissues and make up about 30% of tial or dispensable; they can be synthesized in the body from
the total proteins in the body. Elastin is the protein found in carbon and nitrogen building blocks and need not be present in
tendons and arteries. Keratins are the main proteins in hair. All the food if adequate nitrogen and energy are available. Some
enzymes, hormones and antibodies that are proteins have a amino acids are conditionally essential.These amino acids ordi-
globular structure. Subgroups of globular proteins include the narily are not required in the food except during certain physi-
albumins, histones, globulins and protamines. ologic or pathologic conditions when they may not be synthe-
Conjugated proteins contain amino acids and carbohy- sized in adequate quantities.
drates (glycoproteins), lipids (lipoproteins) or minerals (phos- Although nonessential amino acids can be made from pre-
phoproteins, chromoproteins). Glycoproteins are commonly cursor carbon skeletons, they are just as critical to the makeup
found as components of cell membranes and function to of proteins and are just as essential for metabolic reactions in
modulate enzymes, receptors and immune function and to the body as essential amino acids. Protein is also necessary to
recognize cells (antigen and blood types). Glycoproteins are provide the body with a source of nitrogen for synthesis of other
also components of mucous secretions that act as lubricants in nitrogen-containing compounds including purines, pyrim-
many parts of the body, myelin that surrounds nervous tissue idines, nucleotides, nucleic acids, creatinine, nitric oxide and
and as part of lipoproteins. Lipoproteins transport lipids in some neurotransmitters.
the bloodstream in a water-miscible form to tissues.
Lipoproteins are typically classified into four main categories Metabolism
according to their density: 1) chylomicrons, 2) very low-den- Digestion and Absorption
sity lipoproteins (VLDL), 3) LDL and 4) high-density Dietary proteins must be digested to be absorbed from the GI
lipoproteins (HDL) (Chapter 28). Proteins that contain min- tract. Protein digestion begins in the stomach with the action
erals include hemoglobin (iron), cytochromes (copper) and of the enzyme pepsin in the presence of hydrochloric acid.The
caseins (phosphorus). main end products of gastric protein digestion are mixtures of
Some proteins in the body contain special amino acids that large polypeptides; however, little or no absorption of these
are derived from common amino acids. Collagen contains molecules occurs. In the small intestine, the pancreas and cells
hydroxyproline and hydroxylysine, which are derivatives of pro- lining the small intestine secrete other enzymes (endopeptidas-
line and lysine, respectively.Triiodothyronine and tetraiodothy- es and exopeptidases).These enzymes break the bonds between
ronine (thyroxine) are derived from tyrosine and function as the amino acids of large polypeptides resulting in free amino
hormones and part of the protein thyroglobulin. Gamma-car- acids, dipeptides and tripeptides that can be absorbed across the
boxyglutamic acid is derived from glutamic acid and is key for intestinal wall (Mathews, 1991). Some proteins are less readily
the function of calcium binding in thrombin, which functions digested than others. The rapidity of digestion is influenced by
in blood clotting. Other amino acids such as taurine (a β-amino many factors, including protein structure, processing effects,
acid) and γ-aminobutyric acid function in specific roles in the other nutrients in the meal and the presence of enzyme
body, but are not found as part of proteins. inhibitory factors.
Purines (adenine and guanine) and pyrimidines (cytosine, Absorption of amino acids is a sodium-dependent, active-
thymine, uracil) are other nitrogen-containing molecules that transport process that requires energy (ATP). This process is
form nucleic acids. Nucleic acids (RNA and DNA) carry the mediated by four different carrier systems that transport neutral
genetic information that codes for the primary structure of pro- amino acids, basic amino acids, dicarboxylic amino acids and
teins (the amino acid sequence). imino acids. These separate carrier systems help ensure trans-