The cost of enzyme synthesis in the genetics of energy balance and physiological performance

The study of metabolism has traditionally focused upon factors that influence metabolic rate, at levels of both the metabolic pathway and the whole organism. This paper focuses on the cost, and thereby the efficiency, of metabolic processes. The genotype-dependent cost of enzyme turnover is proposed as a biochemical genetic mechanism for relating genetic variation at single genes to phenotypic variation in quantitative traits of energy metabolism. Decreased costs of maintenance metabolism can accompany artificial selection for increased production (e.g. growth, reproduction, etc.) and lower maintenance is correlated with multiple locus heterozygosity in outbred populations. In both cases, high production has been associated with lower rates of protein turnover. Several factors influence the ATP-equivalent cost of enzyme turnover. These factors are used to calculate the cost of turnover for a single enzyme. This cost can conservatively constitute up to several percent of the total daily mass-specific energy demands of maintenance metabolism. Genetic variants of an enzyme can differ in the cost of turnover. These differences can constitute the basis for metabolic changes associated with artificial selection for production and the metabolic differences that are associated with individual levels of heterozygosity. The metabolic and evolutionary significance of genotype-dependent turnover costs is a function of individual energy balance. The strength of selection against increases in cost will be an inverse function of individual energy balance and is therefore influenced by both environmental and genetic factors.