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. 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