Question

Propionyl-CoA is found within mammalian cells, arising from at least three sources. What are the sources? Elaborate. In 3 enzyme catalyzed steps, the propionyl-CoA is ultimately converted to succinyl-CoA, a process that requires bicarbonate. Imagine that you have an in vitro set-up of ¹³C-labelled propionyl-CoA (label on the middle C of propionyl), unlabelled bicarbonate, and the enzymes. By showing the three relevant equations, indicate the location of the ¹³C-label in each step and in the product succinyl-CoA.   It is NOT necessary to draw any mechanisms, just provide overall equations. Is ¹³C a stable isotope? ¹³C can be detected by 2 distinct methods. Name each. Also, what happens to the succinyl-CoA in an in vivo environment. Where does it go?

Answer 1

1. Three sources of Propionyl-CoA are
   • Beta-Oxidation of odd chain fatty acids, after which it is converted into succinyl CoA and fed into TCA cycle
   • Catabolism of Valine and Isoleucine
   • Metabolism of Threonine
2. 
3. C13 is a stable isotope ( 6 protons, 7 neutrons)
4. C13 can be detected by mass spectroscopy or NMR spectroscopy (Nuclear magnetic resonance)
5. Succinyl CoA is directed to Kreb's Cycle