Protein plays a contributory role as an energy substrate during endurance-type activities. The amino acids (primarily the branched-chain amino acids leucine, isoleucine, valine, glutamine, and aspartate) first must convert to a form that readily enters pathways for energy release. This conversion requires removing nitrogen from the amino acid molecule. In this way, the muscle can directly use for energy the “carbon skeleton” by-products of donor amino acids. Only when an amino acid loses its nitrogen containing amino group can the remaining compound (usually one of the Krebs cycle's reactive compounds) contribute to ATP formation. Some amino acids are glucogenic; they yield intermediate products for glucose synthesis via gluconeogenesis. This gluconeogenic method serves as an important adjunct to provide glucose during prolonged exercise.