|MadSci Network: Biochemistry|
Thanks for the great question Amaya,
I had a great time researching the answer to your question. I got to go to the library and read some 30 year old papers on microfiche, and contacted some of the authors for clarification. Your question applies to the study and application of isotopic fractionation in the field of biogeochemistry.
Okay, here is some background. There are two stable (i.e., non radioactive) isotopes of carbon, known as carbon 12 (12C) and carbon 13 (13C). The difference between these two isotopes is the presence of an additional neutron in the nucleus of the 13C atom, so that this isotope is a little heavier than the 12C isotope. In general there is an overabundance of 12C atoms in comparison to 13C atoms, with a 13C/12C ratio of approximately 1:99.
Your question deals with the interesting observation that this ratio is even lower in organisms than it is in the "inorganic" world, so that organisms are "enriched" in 12C. What is even more interesting is the observation that this enrichment seems to be the result of the action of a few key metabolic enzymes that seem to "prefer" 12C to 13C. That is to say that in the case of most (or at least many) enzymes, there does not seem to be a strong preference for 12C over 13C. Now, the simple answer to your question is, because 12C is less massive than 13C, but the real answer is more complex, and I want to give you some more background in order to explain.
The degree of preference or enrichment for 12C is measured with a statistic known as delta 13C. This value tells you the difference in parts per thousand between the 13C/12C ratio in a given sample and that ratio in the "inorganic" world. So a delta 13C value of -10 would mean that that sample (a piece of kelp, say) had 10 fewer 13C atoms per thousand than we expect from the general ratios in the non-biological world.
Here are some examples of delta 13C values from different types of organisms that convert "inorganic" carbon (e.g., CO2) to "organic" carbon (e.g., glucose).
|Type of organism||delta 13C value|
|C4 photosynthetic plants||-13|
|C3 photosynthetic plants||-28|
|Thermal vent microorganisms||-33|
Now, the organisms on the table each fix carbon from the environment in different ways. The processes of C3 and C4 photosynthesis result in different delta 13C values, and the chemosynthetic fixation of carbon near thermal vents results in an even higher value. The final interesting observation in all of this is that you see the same delta 13C values in organisms that eat C3 or C4 plants as in the plants themselves, so that there isn't much additional enrichment for 12C after the initial event in which the carbon was fixed.
One interesting exception to this is in an enzyme called pyruvate dehydrogenase, which converts pyruvate into acetyl-CoA. This enyzme also demonstrates an isotopic fractionation effect. It prefers pyruvate molecules with a 12C for the second carbon (keto carbon), to pyruvate with 13C keto carbons. The enzyme doesn't seem to care what the other carbons in pyruvate are. This helps us answer your question, because it could be the case that enzymes simply prefer 12C atoms because they are less massive. In this case, you wouldn't necessarily expect that the effect of fractionation would be focused on one particular atom in a pyruvate molecule; you would expect that pyruvate molecules with 3 12C atoms would be prefered to pyruvate with 3 13C atoms.
In fact, the pyruvate dehydrogenase enzyme makes a covalent bond with the pyruvate's keto carbon, and to get back to your question, the reason pyruvate dehydrogenase prefers 12C to 13C is because lower mass 12C atoms are slightly more energetic, or "willing" to participate in reactions. This is generally true for isotopes of any atom. It is easier to break chemical bonds to lower mass isotopes than to the heavier mass isotopes of any element. In example, the biological preference for 12C over the radioactive isotope 14C is almost double the preference for 12C over 13C.
Finally, I want to reiterate that most enzymes don't really seem to care which isotope of carbon is present. In fact, even photosynthetic enzymes will fix 13C CO2 molecules when they aren't given a 12C alternative. The reason we see a preference for 12C in biological systems is because some key carbon fixing and metabolic enzymes are "finicky" or "shrewd" enough to prefer the more energetic 12C atoms.
I hope this answers your question. Here are some references you might want to pursue for more information.
R.E. Criss. 1999. Principles of Stable Isotope Distribution. Oxford U.Press.
Bender, M. M. (1971) Phytochemistry 10, 1239-1244
DeNiro, M. J., and Epstein, S. (1977) Science 197, 261-263
Rau, G.H. and J.I. Hedges. (l979) Carbon-13 depletion in a hydrothermal vent mussel: Suggestion of a chemosynthetic food source. Science 203:648 - 649.
Rau, G.H. (1981) Hydrothermal vent clam and tube worm 13C /12C: Further evidence of non-photosynthetic food sources. Science 213: 338 - 340.
W. C. M. C. KokkeS, Samuel Epsteing, Sally A. LooklI, Greg H. Raull, William FenicallI, and Carl Djerassis (1984) On the Origin of Terpenes in Symbiotic Associations between Marine Invertebrates and Algae (Zooxanthellae) Culture Studies And An Application Of 13C/12C Isotope Ratio Mass Spectometry Journal OF Biological Chemistry 259, 8168-8173
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