fuel during exercise (9,25). Proteins can enter bioenergetic pathways in a variety of places. However, the first step is the breakbown of the protein into its amino acid subunits. What happens next depends on which amino acid is involved. For example, some amino acids can be converted to glucose or pyruvats,some to acetyl-Coa, and still others to Krebs-cycle intermediates, The role of proteins in bioenergetics is summarized in Fig. 3.19.
In summary, the Krebs cycle completes the oxidation of carbohydrates, fats, or proteins; produces Co2; and supplies electrons to be passed through the electron transport chain to provide the energy for the aerobic production of ATP. Enzymes catalyzing Krebscycls reactions are located inside the mitochondria.
Electron Transport Chain The aerobic production of ATP (called oxidative phosphorylation ) occurs in the mitochondria. The pathway responsible for this process is called the electron transport chain (also called the respiratory chain or cytochrome chain). Aerobic production of ATP is possible due to a mechanism that uses the potential energy available in reduced hydrogen carriers such as NADH and FADH to rephosphorylate ADP to ATP. Note that