Structural determinants in the group III truncated hemoglobin from Campylobacter jejuni

Area of Science:

• Biochemistry
• Structural Biology
• Protein Science

Background:

• Truncated hemoglobins (trHbs) are a distinct globin superfamily lineage.
• Phylogenetic analyses identified three groups (I, II, III) within the trHb family.
• Group I and II trHbs possess a simplified 2-on-2 alpha-helical sandwich fold; no structural data existed for group III.

Purpose of the Study:

• To determine the three-dimensional structure of a group III truncated hemoglobin (trHbP) from *Campylobacter jejuni*.
• To investigate the structural conservation and modifications within group III trHbs compared to other groups.

Main Methods:

• X-ray crystallography was used to determine the 2.15-A resolution crystal structure of *C. jejuni* trHbP (cyano-met form).
• Structural analysis focused on the globin fold, heme crevice, and potential ligand pathways.

Main Results:

• The 2-on-2 trHb fold is conserved in group III trHbP, even without previously assumed essential Gly-based motifs.
• Significant modifications were observed in the heme crevice (C-E region, FG helical hinge) and novel proximal surface clefts.
• Unlike groups I and II, *C. jejuni* trHbP lacks evident protein matrix tunnels/cavities.
• The His(E7) side chain exhibits conformational flexibility, potentially gating ligand access to the distal heme site.

Conclusions:

• The study reports the first 3D structure of a group III truncated hemoglobin.
• Group III trHbs share a conserved fold but exhibit unique heme pocket features and lack internal cavities.
• The findings provide insights into the structural diversity and functional adaptations within the truncated hemoglobin family.