Oxygen Transport Proteins

Oxygen Transport

deoxyMyoglobin

oxyMyoglobin

deoxyHemoglobin

• found in vertebrates, Hb used for O₂ transport in blood, Mb for O₂ storage in muscle
• Mb: single protein chain, primarily α-helices
• Hb: four protein chains, each similar in size and function to that of Mb
• deoxyHb and Mb: blue, CN=5 HS Fe(II), heme + axial NHis
• oxyHb and Mb: red, CN=6, heme + axial NHis + bent terminal O₂, often described as antiferromagnetically coupled LS Fe(III)/(O₂^•–)

In deoxyMb and deoxyHb, the large HS Fe(II) sits slightly out the plane of the porphyrin ring. Oxygen coordinates in the vacant axial position, and forms a hydrogen bond to another histidine group that points into the binding pocket. Coordination reduces the radius of the iron (forming either LS Fe(II)(O₂) or LS Fe(III)(O₂^•–) depending on who you ask and whether you believe there's a meaningful difference). This reduces the ionic radius of the Fe, allowing it to move into the ring, pulling the bound histidine group with it. In Hb, this movement of the histidine by over 50 pm induces a reorganization of the protein chains in the other subunits, giving rise to a cooperativity effect, whereby binding of one or two O₂ groups facilitates oxygenation of all four iron centres.

deoxyHemocyanin	oxyHemocyanin	deoxyHemocyanin: biological assembly

• O₂ transport protein found in crustaceans and some molluscs
• deoxyHc: colourless, two CN=3 Cu(I) d¹⁰, each bound by three Nhis groups
• oxyHc: blue, two Cu(II) d⁹, antiferromagnetically coupled through O₂ group
• O₂ binds as μ-η²:η² O₂^2– peroxo

deoxyHemerythrin: active site	oxyHemerythrin: active site	deoxyHemerythrin: biological assembly

• O₂ transport protein found in marine invertebrates
• eight subunits, primarily α-helices, asymmetric diiron binding site
• deoxyHr: colourless, two HS d⁶ Fe(II) bridged by μ-OH
  • one Fe is CN=6: 3xNHis, OGlu, OAsp, μ-OH
  • one Fe is CN=5: 2xNHis, OGlu, OAsp, μ-OH
• oxyHr: red violet, two LS d⁵ Fe(III) bridged by μ-O, antiferromagnetically coupled
  • O₂ binds to unsaturated Fe and abstracts H-atom from OH, forming FeOOH group that remains hydrogen bonded to μ-oxo group

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