Final answer:
During step 3 of the Krebs cycle, isocitrate is formed from citrate through isomerization. Subsequently, isocitrate is oxidized to form α-ketoglutarate, producing NADH and CO2 in the process.
Step-by-step explanation:
In step 3 of the Krebs cycle during aerobic respiration, the 6-carbon compound citrate undergoes an isomerization to form isocitrate. This is so that in subsequent steps, it can be properly oxidized. During the isomerization of citrate to isocitrate, no significant molecule 'enters' as substrate or 'exits' as a product, but it is crucial for the ensuing oxidative steps which will lead to energy production. Following this step, isocitrate is then oxidized and decarboxylated, leading to the formation of a 5-carbon compound, α-ketoglutarate, while reducing NAD+ to NADH and releasing carbon dioxide (CO2) as a waste product. This is the first of several oxidative steps that result in the reduction of electron carriers needed for the electron transport chain where the bulk of ATP is produced.