text: Chemical reactions in biological processes are controlled by enzymes that catalyze the conversion of substrate to product. Since enzymes can alter the transition state structure for reactions, they also change kinetic and equilibrium isotope effects.  Placed in the context of a metabolism, the expression of isotope effects on biomolecules is further controlled by branch points. Different pathways of biosynthesis will use different enzymes, yielding a range of position specific isotope enrichments. This variability allows position-specific isotope measurements to discern multiple biosynthetic pathways from the same metabolic product. Biogeochemists use position specific isotope enrichments from amino acids, lipids, and sugars in nature to interpret the relative importance of different metabolisms.