An Idea to Explore: Using C/H Bond Count to Understand Oxidation-Reduction Reactions in Nutrient Metabolic Pathways

Understanding oxidation-reduction (redox) reactions is essential for studying nutrient metabolism; however, the standard criteria for classification are often difficult to apply. Here, we present a simple method in which the C-C, C═C, and C-H bonds (collectively, C/H bonds) are counted on Kekulé structures of the reactants and products and used to classify a metabolic reaction as follows: a decrease in C/H bond count is an oxidation reaction, an increase is a reduction reaction, and no change in count denotes a non-redox reaction. Similarly, an entire pathway can be classified by comparing the counts on initial and final metabolite structures. For example, glucose → 2 pyruvate (12 → 10 C/H bonds) is an oxidation reaction, whereas glucose → 2 lactate (12 → 12) has no net redox change. The C/H bond count is linked to the degree of reduction (γ) and to the total carbon oxidation number (TCON) through TCON = 4m - γ = 4m - 2 × [C/H bonds] for an m-carbon compound. This method makes three principles readily apparent: nutrient metabolism proceeds mainly by dehydrogenation, decarboxylation is not a redox reaction, and the number of dehydrogenations for complete oxidation can be predicted from the C/H bond count. This method provides a quantitative counterpart to qualitative redox criteria and offers a deductive method of understanding metabolic pathways.