ABSTRACT
The acyl-CoA pathways are initiated by an enzyme that catalyzes the reaction that combines free fatty acid with coenzyme A (CoA) to give an acyl-CoA ester (Fig. 13-1). This reaction proceeds rapidly even with small amounts of nonesterified fatty acid so that it keeps the amount of nonesterified acid very low in healthy tissues. The fatty acid in the acyl-CoA form is the starting point for many metabolic conversions (Fig. 13-1). It may be transferred to an intermediate acylcarnitine that is transported to the part of the cell where it undergoes oxidation to CO2 with the generation of cellular energy, very similar to the way that burning coal in a locomotive can be transformed into work. The oxidation provides energy that allows humans to move heavy objects, plow fields, and build cities. Throughout our history, human labor has been fueled by this sort of oxidative energy, and the high calorie intake of laborers doing heavy work has been aided by the rich caloric content of fats. So long as a high expenditure of energy balanced the dietary intake, the dietary fat was “burned” and did not accumulate.
