Nonruminant Nutrition symposium: Understanding protein synthesis and degradation and their pathway regulations1

Domestication of both animals and plants approximately 13,000 yr ago marked the first major milestone of the evolving animal production, which had advanced ancient humans from a hunter-gatherer society challenged by survival to a prolonged farming society (Diamond, 2002). Modern science and technology has facilitated the second major milestone of developing industrialized intensive animal production systems driven by use of fossil fuels during the last century, especially during the last 2 decades. Although industrialization and globalization are reaching the corners of the earth, current intensive animal production is facing new challenges, including 1) economical and social viability in rural regions faced with increasing labor and living costs; 2) resource and environmental sustainability; and 3) human health management, partly due to an increase in the consumption of animal products. Animal production is, in essence, a mass and energy conversion process using farm animals as a biological converter. Modern intensive animal production systems have increased the speed and scale but have not increased the efficiency very much of this mass-energy conversion. A large proportion (60 to 80%) of the total animal farming operation cost is associated with animal feeding, primarily due to a poor efficiency of utilization of N and energy needed (Fan et al., 2006). At the whole-body level, CP deposition is the net balance among synthesis, degradation, and the fecal gastrointestinal endogenous loss (Fan et al., 2006). Skeletal muscle is the largest body CP pool. Protein synthetic activity of skeletal muscle decreases dramatically during postnatal growth from suckling to weaning and maintains at a low level for most of the duration of production (Reeds et al., 1993). Protein degradation is the principal negative component in the whole body CP deposition equation. As the primary pathway, the ubiquitin-proteasome protein degradation is an ATP-demanding process, costing approximately 300 to 400 mol of ATP for breaking down 1 mol of a typical protein (Benaroudj et al., 2003). The energy cost for synthesizing protein is approximately 0.90 KJ/g, calculated from the pathway stoichiometry summarized by Yang et al. (2008), whereas the energy need for degrading protein is about 0.40 KJ/g of protein with a molecular weight similar to β-casein (24 kD) calculated according to the work by Benaroudj et al. (2003). The increases in postnatal endogenous CP loss are largely from the high protein synthetic activity of the gut and other viscera that can be further augmented under challenged conditions (Fan et al., 2006).

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