This active coenzyme is called the NAD (nicotinamide adenine dinucleotide). It is an important ingredient in metabolism, particularly before proceeding to the Citric Acid cycle. The reaction pathway is very complex. However, the gist is that, the enzyme pyruvate dehydrogenase and coenzyme NAD convert the pyruvate to the starting substance of the Citric Acid cycle, which is Acetyl-CoA.