Protein Synthesis

Protein synthesis is initiated when a signal (e.g., nutrient, hormone, mechanical) to the cell is communicated to the DNA to induce gene expression (transcription),8

resulting in formation of messenger RNA (mRNA). The mRNA is translated into a protein through the process of translation by the ribosomes, which are free in the cytosol or bound to rough endoplasmic reticulum. The process of translation requires a second form of RNA, called transfer RNA, and three distinct steps: initiation, elongation, and termination. Following translation, the nascent protein can be further modified through processes such as glycosylation or degradation (posttranslational modification).

When the entire process of muscle protein synthesis is considered, there is ample evidence that this increases in a similar manner after both endurance910 and resistance1114 exercise. Factors such as the intensity and duration of exercise also have profound effects on gene expression.15 The sites of regulation and how this generalized protein synthetic response is fine-tuned to allow for phenotypical divergence are just becoming unravelled. We have used microarray technology and found that over 200 mRNA species are differentially expressed by only 3 h after endurance exercise,16 and only a minority of these same species are expressed in a similar fashion following resistance exercise.17 Others have found that phosphorylation of proteins such as p70S6k, 4E-BP1, eIF-2B, and AMPK is altered in response to different contraction patterns in skeletal muscle.1819 Collectively, the data show that there are changes at multiple levels (e.g., transcription and translation) within the protein synthetic pathway that simultaneously respond to exercise. It is also likely that the state of training will have a major role in determining the absolute and relative importance of transcriptional and translational control of certain proteins and how this relates to protein synthesis,82021 ultimately modulating the phenotypic response to a given pattern of muscle contraction. Innovative approaches have revealed that muscle conserves the ability to acutely and directionally respond to divergent stimulii, even if its training history is at the opposite end of the metabolic demand spectrum.22

The relationship between exercise and nutrition and the fundamental aspects of gene expression and translation are only now being explored.23-26 For example, low muscle glycogen content causes an induction of IL-6 mRNA content in skeletal muscle,24 which may function as a homeostatic sensor to increase hepatic glucose production.27 During exercise, glucose supplementation, compared with water intake, can attenuate increases in the expression of PDF-4 and UCP-3 — genes involved in metabolic regulation.28 Insulin and amino acids also influence the assembly of the initiation complex required for protein translation.29 Recent evidence suggests that diet and exercise may influence different components of the synthetic pathway, for amino acids altered p70 S6 kinase and eukaryotic initiation factor 4E-binding protein-1 phosphorylation status, while resistance exercise had no effect at the same time point.30 Consequently, nutrition and exercise may alter protein metabolism through independent and possibly complementary pathways. If and how these changes will impact upon protein and amino acid requirements at various phases of the training continuum is not clear. It is theoretically possible that endurance exercise training could impact upon amino acid requirements through increased synthesis of enzymes, capillaries, hemoglobin, and myoglobin (in addition to oxidation). The amino acids for these processes may be derived from an increase in dietary protein intake or an increase in the efficiency of amino acid reutilization.

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