Vanabins as oxygen carriers
Because of the high concentration of vanadium in the blood, it has been assumed that vanabins are used for oxygen transport like iron-based hemoglobin or copper-based hemocyanin. Unfortunately no scientific evidence can be found to back this hypothesis.
The highest concentration of vanadium found so far, 350 mM, was found in the blood cells of Ascidia gemmata belonging to the suborder Phlebobranchia. This concentration is 10,000,000 times higher than that in seawater.[4]
Vanabins accumulate vanadium in the blood cells and produce V(III) species and vanadyl ions (V(IV)) from orthovanadate ions (V(V)), with the use of NADPH as a reducing agent. Vanabins also transport the reduced vanadium species into the vacuoles of vanadocytes (vanadium-containing blood cells). The vacuoles are kept at a very acidic pH of 1.9 (due to sulfuric acid in it), made possible by pumping hydrogen ions into the vacuoles by use of energy intensive H+-ATPase. All the use of NADPH and ATP to collect and maintain the vanadium is extremely energy intensive, unusual for oxygen transporting proteins.
Other oxygen transporting proteins have a very low dissociation constant with their metal prosthetic group and bind these groups tightly. Vanabins on the other hand have a moderate dissociation constant and do not tightly bind vanadium. Most importantly, because of this moderate dissociation constant, vanadium is usually found free-floating and separated from any proteins inside the vacuoles. This is completely different from other oxygen transporting proteins.
Another possible function is that vanabin containing organisms cleave the core out of chlorin-based compounds such as the magnesium in chlorophyll and replace it with their vanadium center in order to attach and harvest energy via light-harvesting complexes. It is believed that the bacteria contained in worm castings, Rhodopseudomonas palustris, do this during its photoautotrophism mode of metabolism. By this information it may be hypothesized that kerogen is the result of photoautotrophic bacteria using vanabin as their method of attaching to chlorophyll.