Science explainer
Erythrocruorin vs hemoglobin.
Both proteins carry oxygen. The difference is where they can do it, and that difference is the reason Synfera builds on erythrocruorin instead of human hemoglobin.
The short answer
Human hemoglobin needs a cell around it. Erythrocruorin does not.
Hemoglobin-based oxygen carriers have historically reused human hemoglobin, then had to compensate for what happens when that protein leaves the red blood cell. Erythrocruorin, the extracellular hemoglobin of annelids, works in circulation without a cell around it, so it starts from a different position.
Side by side
Five differences that matter.
| Trait | Human hemoglobin | Erythrocruorin |
|---|---|---|
| Where it carries oxygen | Inside a red blood cell, which shields and stabilises it | In circulation, outside any cell, as an extracellular hemoglobin |
| Molecular architecture | A small free tetramer once removed from the cell | A large multi-subunit complex assembled from many subunits |
| Behaviour outside the cell | Loses stability, the failure mode that limited earlier HBOCs | Already evolved to function outside a cell |
| Blood typing | Human-sourced supply requires typing and crossmatching | An oxygen carrier is not blood-type restricted |
| Source | Donated human blood | Produced by precision fermentation |
Why it is being pursued now
Genomics made a different starting protein practical.
Annelid erythrocruorin is one of the best characterised extracellular hemoglobins, and it can be produced by precision fermentation rather than collected from donors. Synfera's work on it is preclinical research; we publish progress in the newsroom rather than clinical claims.