Page 183 - Color Atlas of Biochemistry

P. 183

174 Metabolism

Protein metabolism: overview is reincorporated into proteins (protein bio-
synthesis). The body’s high level of protein
Quantitatively, proteins are the most impor- turnover is due to the fact that many proteins
tant group of endogenous macromolecules. A are relatively short-lived. On average, their
person weighing 70 kg contains about 10 kg half-lives amount to 2–8 days. The key en-
protein, with most of it located in muscle. By zymes of the intermediary metabolism have
comparison, the proportion made up by other even shorter half-lives. They are sometimes
nitrogencontaining compounds is minor. The broken down only a few hours after being
organism’s nitrogen balance is therefore pri- synthesized, and are replaced by new mole-
marily determined by protein metabolism. cules. This constant process of synthesis and
Several hormones—mainly testosterone and degradation makes it possible for the cells to
cortisol—regulate the nitrogen balance (see quickly adjust the quantities, and therefore
p. 374). the activity, of important enzymes in order
to meet current requirements. By contrast,
structural proteins such as the histones, he-
A. Protein metabolism: overview
moglobin, and the components of the cyto-
In adults, the nitrogen balance is generally in skeleton are particularly long-lived.
equilibrium—i. e., the quantities of protein ni- Almost all cells are capable of carrying out
trogen taken in and excreted per day are ap- biosynthesis of proteins (top left). The forma-
proximately equal. If only some of the nitro- tion of peptide chains by translation at the
gen taken in is excreted again, then the bal- ribosome is described in greater detail on
ance is positive. This is the case during growth, pp. 250–253. However, the functional forms
for example. Negative balances are rare and of most proteins arise only after a series of
usually occur due to disease. additional steps. To begin with, supported by
Proteins taken up in food are initially bro- auxiliary proteins, the biologically active con-
kendowninthe gastrointestinal tract into formation of the peptide chain has to be
amino acids, which are resorbed and distrib- formed (folding; see pp. 74, 232). During
uted in the organism via the blood (see subsequent “post-translational” maturation,
p. 266). The human body is not capable of many proteins remove part of the peptide
synthesizing 8–10 of the 20 proteinogenic chain again and attach additional groups—
amino acids it requires (see p. 60). These e. g., oligosaccharides or lipids. These pro-
amino acids are essential, and have to be sup- cesses take place in the endoplasmic reticu-
pliedfrom food(seep. 184). lum and in the Golgi apparatus (see p. 232).
Proteins are constantly being lost via the Finally, the proteins have to be transported to
intestine and, to a lesser extent, via the kid- their site of action (sorting; see p. 228).
neys. To balance these inevitable losses, at Some intracellular protein degradation
least 30 g of protein have to be taken up (proteolysis) takes place in the lysosomes
with food every day. Although this minimum (see p. 234). In addition, there are protein
value is barely reached in some countries, in complexes in the cytoplasm, known as pro-
the industrial nations the protein content of teasomes, in which incorrectly folded or old
food is usually much higher than necessary. proteins are degraded. These molecules are
As it is not possible to store amino acids, up to recognized by a special marking (see p. 176).
100 g of excess amino acids per day are used The proteasome also plays an important part
forbiosynthesisordegradedin the liverin in the presentation of antigens by immune
this situation. The nitrogen from this excess cells (see p. 296).
is convertedinto urea (seep. 182) andex-
creted in the urine in this form. The carbon
skeletons are used to synthesize carbohy-
drates or lipids (see p. 180), or are used to
form ATP.
It is thought that adults break down
300–400 g of protein per day into amino
acids (proteolysis). On the other hand, ap-
proximately the same amount of amino acids

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