Glossary of "Plant Biology (1st ed.)" by Graham et al. (2003)
- Nonprotein components such as metal ions that are required by certain enzymes for their function.
Glossary of "The Cell (5th ed.)" by Alberts et al. (2008)
- In organic ion or coenzyme required for an enzyme’s activity.
- Alberts, B., Johnson, A., Lewis, J. Raff, M., Roberts, K. & Walter P. 2008. Molecular Biology of the Cell, 5th ed. -xxxiii + 1268 pp. Garland Science, New York.
- Graham, L. E., Graham, J. M. & Wilcox, L. W. 2003. Plant Biology. -xxxv + 497 pp. Pearson Education, Inc., Upper Saddle River.