Which step in a biosynthetic pathway might have a positive ΔG?

In the context of biosynthetic pathways, a positive ΔG indicates a reaction that requires energy input, often occurring in the direction of unfavorable processes. Specifically, the conversion of malate to oxaloacetate is an important step that typically involves the addition of energy, particularly as it is part of the citric acid cycle and is associated with the requirement for NAD+ as a cofactor, which becomes reduced to NADH.

The reaction of malate to oxaloacetate is endergonic under specific conditions, meaning it doesn't proceed spontaneously without input of energy. This reaction aids in maintaining the overall flow of the cycle, ensuring that metabolic needs are met while also enabling gluconeogenesis—creating glucose from non-carbohydrate sources.

In contrast, the conversion of glucose to glucose 6-phosphate, citrate to succinyl-CoA, and oxaloacetate to citrate involve transitions that have more favorable ΔG values because they are often coupled with ATP hydrolysis or occur in pathways that are energetically favorable under physiological conditions. These steps typically release energy, making them easier to drive forward. Thus, in understanding the requirements for energy and the thermodynamics of each reaction, the malate to oxaloacetate conversion is characterized by