Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

III. Metabolism of Proteins
119
C . Physical Classification
Proteins can be classified by physical properties and behavior,
by their size (relative molecular mass, M r ), or by the
charge on the protein. The charge on a protein results from
a combination of the acidic and basic groups on free side
chains of the amino acids of the protein and is dependent
on the pH of the aqueous environment. For every protein,
there is a specific pH where the protein has an equal number
of negative and positive charges on its side chains and
the protein has a net charge of zero. This is the isoelectric
point of the protein (pI). The higher the proportion of basic
amino acids, such as lysine or arginine, the higher the pI of
the protein will be, whereas with more acidic amino acids,
such as glutamate or aspartate, the protein will have a low
pI. The proportion of aromatic amino acids tyrosine, tryptophan,
or phenylalanine contained by a protein influences its
spectral properties because these amino acids absorb light at
280 nm, which can be measured in a spectrophotometer. The
spectral property can also be influenced by factors such as
the presence of heme groups and the binding of metal ions.
The proportion of hydrophobic amino acids defines the
hydrophobicity of a protein, which can be predicted from
the primary sequence. Chemical composition in terms of
the primary structure and the physical properties of proteins
that have been sequenced are available from online databases
such as the UniProt database at www.ebi.uniprot.org.
III . METABOLISM OF PROTEINS
A . General
The metabolism of nitrogenous compounds in animals is
largely related to the processes of anabolism and catabolism
of amino acids and proteins. Proteins in the diet are broken
down by protease digestion to yield free amino acids and
small peptides, the latter being finally degraded in the intestinal
cells during absorption. The products of protein digestion
enter the portal vein as amino acids. In the healthy animal,
an equilibrium is established between intake and synthesis
of amino acids, on the one hand, and breakdown and excretion
of excess nitrogenous material, in the form of urea, on
the other. Excessive loss of nitro-genous material can occur
in illness because of cellular breakdown, lactation with production
of milk protein, and in urinary or gut losses. During
growth, pregnancy, and recovery from disease, there is a positive
nitrogen balance as amino acids and other nitrogenous
compounds are supplied to meet the body’s requirements.
B . Synthesis of Proteins
Proteins are made from amino acids in the cytoplasm of
cells when the appropriate mix of amino acids is present.
Among the 20 naturally occurring amino acids found in
TABLE 5-1 Natural Amino Acids
Essential Amino Acids
Histidine Lysine Threonine
Isoleucine Methionine Tryptophan
Leucine Phenylalanine Valine
Nonessential Amino Acids
Alanine Cysteine Proline
Arginine Glycine Serine
Asparagine Glutamate
a
Tyrosine
Aspartate
Glutamine
a
By conversion from phenylalanine.
protein, nearly half cannot be synthesized by mammalian
cells. These are the essential amino acids that have to be
obtained in the diet ( Table 5-1 ). The nonessential amino
acids can be synthesized by transamination reactions in
which the amino group of glutamate is transferred to a
carbon skeleton in the form of an α -ketoacid. An example
of this is the action of alanine transaminase, which catalyzes
the transfer of the amino group of glutamate to the
α -ketoacid, pyruvate, with the formation of alanine and
α -ketoglutarate. Alanine transaminase (ALT; EC 2.6.1.2)
is an important diagnostic enzyme, used as a marker of
liver damage (Chapter 12) in small animals. The nonessential
amino acids can be synthesized in animals from
components of the central metabolic pathways, whereas
the essential amino acids have to be present in the diet.
However, in ruminants, the symbiotic relationship with
ruminant microbes allows production of the full range of
amino acids so that these species do not require all the
essential amino acids in their dietary intake.
The intricate process of synthesis of protein in the
ribosomes of the rough endoplasmic reticulum, under
the instruction of messenger ribonucleic acid (mRNA),
is a major part of the discipline of molecular biology and
will not be described here in detail as authoritative texts
are devoted to the subject ( Alberts et al. , 2002 ). The primary
structure of the protein is determined by the gene
sequence of nuclear DNA on a chromosome in the nucleus.
The genetic code, which is the sequence of nucleotides in
DNA (adenine, cytosine, guanine, thymine), controls the
sequence of amino acids in the protein. During protein synthesis,
the code is transcribed from DNA to mRNA, which
moves from the nucleus to the ribosomes in the cytoplasm.
Here, specific amino acids are added to the growing peptide
chain following linkage to a specific transfer RNA
(tRNA). The specificity of production of the amino acid
chain during this process of translation is dependent on the
triplet of nucleic acids in the mRNA (codon) binding accurately
to the anticodon of the tRNA. By this means, the