Energy costs of protein and fatty acid synthesis

Abstract

The heat production of growing animals can be divided statistically into two components; one, a function of bodyweight , the so-called maintenance heat production, the other a function of the rate of growth and of energy intake above that required for energy equilibrium. The second of these two components is presumably, at least in part, a consequence of the metabolic transformations which result in the deposition of body tissue (both lipid and protein) and is the subject of this paper. The growth of an animal is the resultant of many separate but interconnected reaction sequences. We will concentrate upon two pathways, namely protein and fatty acid synthesis, and the contribution that these make to the energy expenditure of young growing animals. The 'nutritional' energy costs of these pathways of nutrient utilization can be calculated from their stoichiometry (Table I ; Blaxter, 1969; Millward et al. 1976). The calculation involves consideration of at least three factors; (I) the energy cost of formation of the bonds in the macromolecule, (2) the energy cost of forming the monomers from which the final molecule is synthesized and (3) the cost of the synthesis of the ATP and reduced pyridine nucleotides which are required for the process of polymerization. It should be noted that the energy cost so calculated is a minimal one. It will be higher if the rate of synthesis of the end-product exceeds the rate of accretion and if accelerated rates of protein and fat deposition also involve the activation of other energetically wasteful processes. The former of these two factors can be illustrated by a consideration of the energy cost of protein synthesis. Nitrogen and energy balance, heat production and protein synthesis in the whole body have been measured simultaneously in growing pigs (Reeds et al. 1980, 1981). In these experiments the rate of growth was manipulated in three ways; (I) Table I. The energetic efJiciency of triacylglycerol synthesis from different substrates* Heat production (kJ/mol synthesized) Fatty acids + triacylglycerolt 480 Amino acids-+ triacylglycerol $ 12767 Carbohydrate + triacylglycerol 6104 *After McGilvery (1970) and Millward et al. (1976). ?Assuming it to be tripalmitylglycerol. $Assuming the division of animo acid metabolism between glucose and ketone body synthesis given by McGilvery (1970).