Synthesis and Degradation of Proteins in Pigs

Abstract

Growth of animals is the complex result of competition between anabolic and catabolic process, which implies constant changes and remodeling through synthesis of new proteins and breakdown of existing proteins (Jobgen et al. 2006; Tan et al.,2009). Together, these processes are called protein turnover and produce muscle growth or hypertrophy when synthesis is greater than breakdown and muscle wasting when synthesis is less than breakdown (Norton and Layman 2006). Protein turnover requires large amounts of ATP. However, this costly metabolic cycle fulfills key obligatory functions, including protein homeostasis, cell turnover, removal of aged and damaged proteins, synthesis of new proteins like heat-shock and immunological proteins, gluconeogenesis from amino acids, wound healing, tissue repair, adaptation to nutritional and pathological alterations, and immune responses (Wu 2009). In pigs, although the protein syntheses increase in all tissues, the greatest response occurs in skeletal muscle in response to feeding stimulation. The elevated postprandial protein synthesis in skeletal muscle of pigs therefore increases the protein deposition during the post-absorptive period allowing growth and development. A sharp increase of circulating glucose, insulin, and amino acids, especially some nutritional indispensable amino acids, is observed after meal (Yin et al. 2010, 2011), in accordance with higher protein deposition in pigs (Drew et al. 2012). The mechanism responsible for the stimulation of protein synthesis by feeding was therefore focused on the roles of postprandial circulating glucose, insulin, and amino acids. The protein degradation is a process where proteins are broken into smaller peptides as well as free amino acids, the latter being either reused for new protein synthesis or further degraded into several metabolites, several of them being able to generate ATP for energy use. In addition, the protein degradation plays important roles in animal physiological process, especially in the cellular signal transduction system as well as in the maintenance of the integrity of the proper folded state of protein.