Earthworms build a huge version of hemoglobin to carry oxygen
Why So Big?
Forms and Functions
Exploring the Structure
Erythrocruorin (PDB entry 2gtl)
Structures of erythrocruorin and other giant hemoglobins reveal that they are built with a similar hierarchical structure. The basic unit is composed of 12 hemoglobin chains, which associate into 6 dimers, which are then assembled into a hollow shell with three-fold symmetry. In the tubeworm hemoglobins, two of these shells come together to form the spherical complex. In the larger earthworm erythrocruorin, three linker chains are inserted in the shell. The linker chains have a long helical portion that associates into a trimer, and globular head group that holds onto the twelve globin chains. This functional assembly--a trimer of linker chains and a dozen globins--is termed a protomer. The entire complex is built of 12 of these protomers. To take a closer look at this protomer, and the many disulfide bonds that help to hold it together, click on the image for an interactive Jmol.
Topics for Further Discussion
- The globin chains in these giant hemoglobins are very similar to our own hemoglobin and myoglobin. You can compare these using the Structure Comparison tool at the RCSB PDB.
- The interaction between the subunits in erythrocruorins, and many other smaller hemoglobins from mollusks, echinoderms and segmented worms, is quite different than the arrangement in our hemoglobins. Compare the structures, and notice that the invertebrate hemoglobins often associate with the hemes of neighboring chains in close proximity.
March 2013, David Goodselldoi:10.2210/rcsb_pdb/mom_2013_3