Protein and Amino Acid Metabolism
In contrast to the case of lipids and carbohydrates, no spe-
cial storage forms of either the nitrogen or the amino acid
components of proteins exist. Dietary protein in excess
of the requirement is catabolized to provide energy and
ammonia, a toxic metabolite that is converted to urea in
the liver and excreted by the kidneys. All body proteins
serve a specific function (e.g., structural, catalytic, trans-
port, regulatory) and are potential sources of carbon for
energy production.
Proteins constantly undergo breakdown and synthesis.
During growth, even though there is net deposition of pro-
tein, the rates of synthesis and breakdown are increased.
Total protein turnover in a well-fed, adult human is esti-
mated at about 300 g/day, of which approximately 100 g
is myofibrillar protein, 30 g is digestive enzymes, 20 g is
small intestinal cell protein, and 15 g is hemoglobin. The
remainder is accounted for by turnover of cellular pro-
teins of various other cells (e.g., hepatocytes, leukocytes,
platelets) and oxidation of amino acids, and a small amount
is lost as free amino acids in urine. Protein turnover rates
vary from tissue to tissue, and the relative tissue contri-
bution to total protein turnover is altered by aging, dis-
ease, and changes in dietary protein intake. Several pro-
teins (e.g., many hepatic enzymes) have short turnover
times (less than 1 hour), whereas others have much longer
turnover times (e.g., collagen >1000 days). Turnover of
myofibrillar protein can be estimated by measurement of
3-methylhistidine in the urine. Histidyl residues of actin
and myosin (Chapter 21) are released during catabolism
of these proteins and are excreted in the urine.
Protein turnover is not completely efficient in the re-
utilization of amino acids. Some are lost by oxidative
catabolism, while others are used in synthesis of non-
protein metabolites. For this reason, a dietary source of
protein is needed to maintain adequate synthesis of pro-
tein. During periods of growth, pregnancy, lactation, or
recovery from illness, supplemental dietary protein is re-
quired. These processes are affected by energy supply and
hormonal factors. An overview of amino acid metabolism
is presented in Figure 17-1.
Essential and Nonessential Amino Acids
Plants and some bacteria synthesize all 20 amino acids
(see also Chapter 2). Humans (and other animals) can
synthesize about half of them (the nonessential amino
acids) but require the other half to be supplied by the
diet (the essential amino acids). Diet must also pro-
vide a digestible source of nitrogen for synthesis of the
nonessential amino acids. The eight
essential amino acids
are isoleucine, leucine lysine, methionine, phenylalanine,
threonine, tryptophan, and valine. In infants, histidine (and
possibly arginine) is required for optimal development
and growth and is thus essential. In adults, histidine is
nonessential, except in uremia. Under certain conditions,
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