14 Jul 2014

The Aconitase Enzyme Mechanism

In-Class Activity

Submitted by Laurel Goj Habgood, Rollins College
Categories
Prerequisites: 
Corequisites: 
Topics Covered: 
Subdiscipline: 
Description: 

This learning objective focuses on the enzyme aconitase.  The iron-sulfur cluster is used to regulate iron in the cell and isomerize citrate for energy – two very different mechanisms.  The activity consists of an introduction to the enzyme and a student discussion on the mechanism of the isomerization of citrate to isocitrate; starting in a small group setting followed by a class debriefing.  

Learning Goals: 
  1. The student will be able to recognize that the iron-sulfur cluster does not do redox chemistry, the most common function for hemes.
  2. The student will provide the mechanism of citrate isomerization using electron-pushing arrows.
  3. The student will consider how the iron-sulfur cluster in aconitase might interact with the citrate during the isomerization process.
Equipment needs: 

None.

Implementation Notes: 

I use this activity in my inorganic chemistry class, which is the foundational ACS inorganic course typically taken by students in the spring of their junior year in tandem with our foundational biochemistry course.  There are three class days at the end of the semester allotted for bioinorganic chemistry.  The first day is an introduction to the subfield and heme chemistry.  The activity is used to open the second class to show non-heme functions of iron and then progress to copper in biological systems.  Students are not expected to prepare or read anything prior to the activity.  For those interested to learn more about aconitase I mention that it is a former molecule of the month in the protein data bank.

The first slide provides essential talking points for a brief introduction of the enzyme for the students.  The second slide shows a bare bones scheme of the mechanism, the two amino acids involved in the isomerization, and a representation of the iron-sulfur cluster which are my adaptations from published images.  The students are divided into small groups and instructed to use electron-pushing arrows to complete the organic portion of the mechanism and speculate about how the iron is involved in the mechanim.  After about five minutes the groups are disbanded and the activity is debriefed.  A student volunteer is asked to come to the board to fill in the electron-pushing arrows and then we discuss the role of the iron.  The notes section of the slide provides resources to aid the faculty member in the debriefing.

Time Required: 
10-15 minutes in class
Evaluation
Evaluation Methods: 

This activity was not formally assessed but used as part of the participation grade for students.  Participation counts for 10% of their overall grade in the course.

Evaluation Results: 

The activity starts with students pairing up or forming small groups of three to work through the organic mechanism and discuss iron’s role in the process.  After five minutes we return to the class as a whole and the activity debriefed by having students contribute their answers with guidance on my part for the correct and incorrect answers.  Students initially struggled with the acid/base chemistry of amino acids so I now provide the protonated/deprotonated forms pertinent to the mechanism.  Now all the groups are able to complete the organic mechanism.  The two most common speculations for iron interaction amongst the students are the coordination of the deprotonated oxygen on a terminal carboxylic acid and coordination of the iron to the olefin.  The literature currently supports the former interaction.

Creative Commons License: 
Creative Commons Licence

The VIPEr community supports respectful and voluntary sharing. Click here for a description of our default Creative Commons license.