Effects of assumptions on estimating energetic efficiencies in lactating dairy cows

Abstract Text:

Theoretical estimates of efficiencies of metabolic and physiological processes have proven useful in teaching undergraduate and graduate courses in animal metabolism and bioenergetics. To maintain accurate numbers, new understandings have been applied to various biochemical pathways in ruminants.  Further understanding of the mammalian ATP synthase, costs of membrane transport, electron transport, and oxidative phosphorylation have improved the accuracy of estimated theoretical efficiencies of energy metabolism. Advances include identifying the costs of: 1) the malate-aspartate shuttle for transporting protons and electrons into the mitochondria, 2) pyruvate transport into the mitochondria, and 3) counter transport of ATP and GTP out of the mitochondria with the concomitant cost of transporting ADP, GDP, and inorganic phosphorous into the mitochondria. As currently accepted, electron transport results in 10 and 6 protons pumped from the matrix to the intermitochondrial space when the electrons are derived from NADH + H⁺ and FADH₂, respectively. The discovery that bovine heart ATP synthase requires the flow of 8 protons from the intermitochondrial membrane space into the mitochondrial matrix through the ATP synthase to produce 3 ATP is critical to estimating energetic efficiency in mammalians. Consequently, 2.7 ATP are produced from each NADH + H⁺ formed in the mitochondria, 2.5 ATP from each NADH + H⁺ formed in the cytosol, and 1.6 ATP from each mitochondrial FADH₂ when each is coupled to oxidative phosphorylation. Mitochondrial GTP formed in the TCA cycle is equivalent to 0.7 ATP when GTP is transported out of the mitochondria. Energy available from ATP hydrolysis varies from 32 to 52 kJ/mole of ATP with an average of 42 kJ/mol. Using these data, theoretical efficiencies and net ATP production/cost were calculated and compared to those of Baldwin [Journal of dairy science 51(1): 104-111. (1968)] (Table 1). These values demonstrate the importance of applying current research and ideas to create accurate values of theoretically estimated metabolic and physiological pathways.

Table 1. Theoretically estimated efficiencies and net ATP production/cost for various biochemical pathways

	Current		Baldwin, 1968
Pathway	ATP Produced	Efficiency	ATP Produced	Efficiency
Propionate to glucose	-4.7	-	-4	-
Glucose to CO2 and H2O	32.9	49%	38	56%
2 Propionate to CO2 and H2O	28.2	38%	34	46%
Fat turnover	45.7	36%	-17	-
Milk lactose synthesis	-	76%	-	78%
Milk protein synthesis (100 g)	-	77%	-	82%
Milk fat synthesis	-	71%	-	72%
Total milk synthesis	-	75%	-	76%

Keywords: energy, metabolism, ATP