Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Krebs Cycle Oxygen Source

Name: Matthew
Status: student
Grade: 9-12
Location: NH
Country: USA
Date: Spring 2013

My biology class is currently learning about the Krebs cycle and how a cell acquires energy. After studying the cycle, I have noticed that during the transition from Succinyl-CoA to Succinate, there is an extra oxygen atom added into the cycle and the is no origin of how it got there. Succinyl-CoA has three oxygens in it (excluding the oxygens found in CoA) and succinate has four. Where did this oxygen originate from and how did it get into the cell? We are currently using Raven's Seventh Edition Biology as our text.


That's a fantastic question! Kudos for noticing this important detail.

As it turns out, the reaction that converts succinyl CoA to succinate has an intermediate structure that you probably didn't learn about. An inorganic phosphate group first attacks the terminal carbon of the succinyl molecule and causes the CoA group to be displaced; this leaves succinyl phosphate as an intermediate. The enzyme then catalyzes a reaction where the phosphate group - missing an oxygen, which it leaves behind on the succinate - is transferred to a histidine residue on the enzyme. The succinate then leaves the active site of the enzyme, and the phosphate group is transferred to a nucleoside diphosphate such as ADP or GDP, turning it into ATP or GTP.

If you're interested in more details of how this reaction works, I would encourage you to search for succinyl coenzyme A synthetase; you will find some useful diagrams of the mechanism of action.

S. Unterman Ph.D.

Hi Matthew,

Thanks for the question. Succinyl Co-A reacts with water (H2O) to form Succinate. This reaction is called a hydrolysis reaction in which a thioester is converted into a carboxylate. The missing oxygen atom you are referring to is from water.

I hope this helps. Please let me know if you have more questions. Thanks Jeff

Click here to return to the Molecular Biology Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 223
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: November 2011
Weclome To Newton

Argonne National Laboratory