Vanabin

Vanabins are found almost exclusively in the blood cells, or vanadocytes, of some tunicates (sea squirts), including the Ascidiacea.

These organisms are able to concentrate vanadium from the surrounding seawater, and vanabin proteins have been involved in collecting and accumulating this metal ion.

[3] 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.

All the use of NADPH and ATP to collect and maintain the vanadium is extremely energy intensive, unusual for oxygen transporting proteins.

Most importantly, because of this moderate dissociation constant, vanadium is usually found free-floating and separated from any proteins inside the vacuoles.