Download Balance Between Protein Synthesis and Degradation

Balance Between Protein Synthesis and Degradation
Brandon Senne
Oklahoma State University
Proteins within the body are constantly degraded and resynthesized in a normal
process termed protein turnover. Dietary intake if nitrogen in the form of amino acids is
related to excretion of nitrogen by the state of nitrogen balance. Organisms have a
positive nitrogen balance if protein synthesis is increased over degradation whereas
negative balanced is observed when amino acids used for tissue building and energy are
not replaced. In eukaryotes nitrogen balance can be affected by several catabolic
conditions, such as disease, starvation, trauma, metabolic acidosis, composition of diet,
and stage of growth.
Regulation of Protein synthesis
Synthesis is regulated by hormones, cytokines, and growth factors while various
inhibitory factors act against these compounds. A review of literature indicates protein
phosphorylation plays a major role in both of these regulatory molecules. Figure 1 shows
that protein synthesis involves a complex series of protein:protein and protein:RNA
interactions culminating in the formation of peptide bonds between amino acids, as
encoded by the mRNA being translated [1]. There is some evidence of regulation at the
elongation phase [2], however, most data shows protein synthesis is regulated primarily
at the level of initiation [3,4]
EIF2/GTP
EIF2/GTP/Met-tRNA
40S
eIF3
43S initiation complex
mRNA + eIF4E
+ eIF4G
GDP
EIF4A eIF4B unwinding
and scanning
eIF2B
GTP
48S initiation complex
60S
eIF2/GDP
80S initiation complex
Elongation
eIF2B: recycling of eIF2 regulated by phosphorylation of eIF2 and eIF2B
mRNA + eIF4E + eIF4G: mRNA binding regulated by phosphorylation of
binding proteins
Elongation: elongation regulated by phosphorylation of eIF2
Figure 1. Regulation of protein synthesis initiation
Since protein synthesis declines with food depravation, considerable interest has
been given to other growth factors and hormones such as insulin-like growth factor I
(IGF-I), growth hormones (GH), and insulin for treatment of protein degenerating
medical conditions. It is understood that insulin is prominent in controlling fat and
carbohydrate metabolism and it has been demonstrated that insulin can stimulate protein
synthesis and inhibit degradation in vitro [5]. In growing rats, muscle protein synthesis in
the fed state is controlled by the rise in plasma insulin as well as sensitivity of the muscle
to insulin brought about by the amino acid leucine. Several experiments [6,7, Garlick]
performed with growing rats show that stimulation of muscle protein synthesis by feeding
is regulated by an increase in insulin secretion along with a leucine induced increase in
the sensitivity of the muscle to insulin. In adult rat, however, the effect of insulin is
reduced considerably indicating stage of growth as a key part of protein regulation.
Adult humans are similar in that protein synthesis is not affected by insulin. Therfore, in
adult species it is accepted that accretion of muscle protein during feeding is regulated
more by depression of degradation than by increase in synthesis [7]. An exception to
these findings is the mouse. Muscle protein synthesis is restored with feeding after an
overnight fast [8] (Figure 2)
6
5
Protein
Synthesis
%/day
4
3
2
1
0
Fasted
Fed
Fasted Insulin Fasted
IGF
Figure 2. Rates of muscle protein synthesis in skeletal muscle of adult mice
Figure 2 shows an increase in synthesis with IGF-I as well. This has been documented in
human subjects also [9]. IGF-I causes positive nitrogen balance by both inhibition of
degradation and stimulation of synthesis. GH also stimulates synthesis but does not
affect degradation which has made this compound the focus of treatment for patients
suffering negative nitrogen balance [7].
Regulation of protein degradation
Eukaryotic cells control protein degradation by both lysosomal and ATP dependent
mechanisms [10]. The majority of cellular proteins are catabolized through an ATPrequiring, ubiquitin-proteasome pathway mediated by the multiprotein complex 26S
proteasome [11] (figure 3). This pathway has been identified as the mechanism causing
negative nitrogen balance under catabolic conditions ( i.e., fasting, diabetes, cancer, etc.).
Inhibitors of the ubiquitin-proteasome pathway block excessive proteolysis [12] as
further evidence that this is regulates excessive proteolysis. The ubiquinated protein is
degraded in an ATP-dependent process.
Peptides
E1 UbiquitinActivation Enzyme
Recycled
Ubiquitin
Ubiquitin
E1
Protein Degradation
+E2 & E3
Conjugating Enzymes
26S Proteasome
Ubiquitin-protein Conjugate
Figure 3. Schematic representation of the ubiquitin-proteasome pathway.
Degradation of a protein begins when it is targeted for destruction by a ubiquitin
molecule. Which proteins become ubiquinated depends largely by its amino–terminal
residue [10]. This underlying cause of regulation has been highly conserved through
millions of years of evolution and across many different species.
Factors causing increased degradation are hormones and cytokines which
both trigger muscle proteolysis [12]. Hence, numerous experiments have shown that
muscle proteolysis does not increase without the presence of glucocorticoids [12] and the
primary factor opposing glucocorticoids is insulin. As mentioned previously, in the fed
state insulin will suppress degradation and therefore patients in negative nitrogen balance
due to fasting or diabetes require two regulatory signals; glucocorticoids and a decrease
in insulin production. Negative balance due to trauma (i.e., cancer, toxins, burns) results
in the release of cytokines which, along with glucocorticoids, stimulate the ubiquitinproteasome pathway in muscle [13].
Negative nitrogen balance occurs when amino acid catabolism causes loss of
control over protein turnover. In mammals, protein synthesis and degradation is precisely
controlled and under normal conditions people are near equilibrium. In a normal adult
(70 kg) approximately 280 g of protein is synthesized and degraded every day, an amount
that substantially exceeds the turnover of plasma proteins (Figure 4)[13].
Cell Proteins (5.8 kg)
Ribosome
Plasma Proteins
0.5 kg
Free Amino Acid
Pool ~ 62 g
Amino
Acids
Proteins
Proteasome
Figure 4. Magnitude of daily turnover of cellular and plasma proteins
References
1. Morley, S. J. (1996) In: Protein phosphorylation and cell growth regulation.
Harwood academic, U.K. 197-224.
2. Proud, C. G. (1992) Protein phosphorylation in translational control. Curr. Top. In
Cell. Reg. 32:243-369.
3. Rhoades, R. E. (1993) Regulation of eukaryotic protein synthesis by initiation
factors. J. Biol. Chem 268:3017-3020.
4. Voorma, H. A., Thomas, A. M., and Van Heugten, H. A. (1994) Mol. Biol. Rep.
19:139-146.
5. Fulks, R. M., LI, J. B., and Goldberg, A. L. (1975) J. Biol. Chem. 250:290-298.
6. Garlick, P. J., and Grant, I. (1988) Biochem. J. 254:579-584.
7. Garlick, P. J., McNurlan, M. A., Bark, T., Lang, C. H., and Gelato, M. C. (1998)
J Nutr. 128:356-359.
8. Svanberg, E., Zachrisson, H., Ohlsson, C., Iresjo, B. M., and Lundholm, K. G. (1996)
Am. J. Physiol. 270:E614-E620.
9. Barnett, E. J., and Gelfand, R. A. (1989) Diabetes Metab. 5:133-148.
10. Stryer, L. (1988) Biochemistry. 3rd Ed. W. H. Fremman and Co., New York p 795797.
11. Voet, D., and Voet, J. (1995) Biochemistry. John Wiley & sons, New York p 10101013.
12. Mitch, W. E. (1997) Am. J. Clin. Nutr. 67:359-366.
13. Mitch, W. E. (1996) N Engl. J. Med. 355:1897-1905.