Protein turnover synthesis

When older proteins are broken down in the body, they must be replaced. This concept is called protein turnover, and different types of proteins have very different turnover rates. Protein synthesis occurs during the process of translation on ribosomes. Protein breakdown occurs generally in two cellular locations ... 

The quantitative importance of each pathway varies from one tissue to another and from one protein to another. Although hydrolysis of the peptide bonds does not involve ATP, the various processes of protein degradation require considerable expenditure of energy, possibly more than is required for protein synthesis. It is not suprising, therefore, that protein turnover contributes at least 20% to resting energy expenditure (basal metabolic rate). 

The biosynthetic incorporation of amino acids into proteins makes these metabolites valuable endogenous tracers for the characterization of protein turnover. Of the naturally occurring amino acids, administration of a bolus dose of pH]leucine is widely used as a tracer in kinetic investigations of protein synthesis and secretion. 

The amount of material (e.g., macromolecule or metabolite) metabolized or processed per unit time. 2. The balance between synthesis and degradation of a biomolecule. 3. One complete turn of a reaction cycle. See Turnover Number Protein Turnover Pulse-Chase Experiments Exponential Decay... 

NUCLEAR MAGNETIC RESONANCE PROTEIN SYNTHESIS INHIBITORS PROTEIN TURNOVER N-END RULE LEUCINE KINETICS PROTEASE La... 

Protein turnover is a natural process resulting from the balance between degradation and synthesis. 

During the normal synthesis and degradation of cellular proteins (protein turnover Chapter 27), some amino acids that are released from protein breakdown and are not needed for new protein synthesis undergo oxidative degradation. 

Protein turnover results from the simultaneous synthesis and degradation of protein molecules. In healthy adults, the total amount of protein in the body remains constant because the rate of protein synthesis is just sufficient to replace the protein that is degraded. 

Rate of turnover In healthy adults, the total amount of protein in the body remains constant, because the rate of protein synthesis is just sufficient to replace the protein that is degraded. This process, called protein turnover, leads to the hydrolysis and resynthesis of 300 to 400 g of body protein each day. The rate of protein turnover varies widely for individual proteins. Short-lived proteins (for example, many regulatory proteins and misfolded proteins) are rapidly degraded, having half-lives measured in rrh-utes or hours. Long-lived proteins, with half-lives of days to weeks, constitute the majority of proteins in the cell. Structural proteins, such as collagen, are metabolically stable, and hare half-lives measured in months or years. 

An acceleration of protein turnover by thyroxine also has been shown, implying that the hormone may alter various processes by a specific effect on synthesis of certain key proteins Involved in enzymatic reactions, Thus, not only does thyroxine increase the rate of formation of new protein material, hut it also may be responsible for the transformation of non-en/.ymalically active protein Into protein with enzymatic activity. The hormone has also been shown to be capable of acceleration of the synthesis of urea cycle enzymes and probably is essential for the production of a... 

The relative roles played by adjusting the rate of enzyme synthesis versus changing the rate of protein turnover are not known in a general sense, albeit both processes are likely... 

It was just stated that protein turnover has anabolic and catabolic arms. In a subject in a steady metabolic state, these are exactly equal. It may then be of interest to determine the absolute rates of protein synthesis/degradation. Individual protein turnover rates may be expressed in terms of half-lives (tiy) or fractional catabolic rates or simply in terms of grams protein synthesized and degraded per unit time. The same parameters can be derived for whole-body protein turnover. 

As the model suggests, the dietary need for amino acids is determined by the rates of depletion of the free amino acid pool by oxidation or synthesis of protein. During steady state conditions, the contribution to the free pool from dietary intake and protein breakdown should be exactly balanced by the flux out of the pool to synthesis and oxidation. Any condition that increases deposition of protein in the body or the rate of amino acid oxidation should produce an increased need for protein. For example, muscle hypertrophy is dependent on a positive balance of the protein turnover process. If synthesis of protein exceeds the catabolism of protein, then muscle mass will increase and the free amino acid pool would be depleted. Thus, a net increase in protein requires an increase in intake or a decrease in oxidation. Likewise, the same arguments hold for an increase in oxidation of amino acids. 

Exercise is known to have acute catabolic effects on muscle protein turnover. During exercise protein snythesis is depressed which leads to protein catabolism. However, the impact of a relatively short exercise bout on 24-hour protein needs is unclear. Anaerobic exercise can produce hypertrophy of specific muscles depending on the type of training utilized. The hypertrophy is due to a positive balance in protein turnover which appears to be produced by an increase in the rate of protein synthesis after exercise. The increased need for protein during anaerobic exercise is unlikely to be more than 7 grams per day. 

Laurent, G.J., Sparrow, M.P. 4 Mlllward, D.J. (1978) Muscle protein turnover in the adult fowl II. Changes in rates of protein synthesis and breakdown during hypertrophy of the anterior and posterior latissimus dorsi muscle. Biochem. J. 176, 407-417. 

Protein turnover is an important process in living systems (Chapter 23). Proteins that have served their purpose must be degraded so that their constituent amino acids can be recycled for the synthesis of new proteins. Proteins ingested in the diet must be broken down into small peptides and amino acids for absorption in the gut. Furthermore, as described in detail in Chapter 10. proteolytic reactions are important in regulating the activity of certain enzymes and other proteins. 

The body contains approx. 14,000 g protein in total. There is a 24-hour turnover of 600-700 g of the amino acid pool. The musculature has the highest absolute rate of protein synthesis. The protein synthesized here is retained for exclusive use in the muscles. In relation to its weight, the liver generates more protein than the musculature. The synthesis rate in the liver amounts to 120 g/ day, whereby 70-80% of these proteins are released by the hepatocytes, so that only 20-30% remain available for their own use. Plasma protein turnover is 25 g/day, that of the total tissue protein approx. 150 g/day. Amino acid turnover and protein synthesis proceed rapidly and continuously. 

In cirrhotic patients, a reduced synthesis rate of most proteins is found at an early stage, with albumin synthesis being the least compromised factor at first. Fat storage, muscle mass and protein turnover are reduced. This ultimately leads to catabolism (so-called stress metabolism) and increasing muscular atrophy (so-called wasting syndrome). The latter condition can also result from sympathicotonia with elevated catecholamine values similarly, decreased values of IGFl inhibit the formation of muscle tissue. 

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