Structural determinants in the group III truncated hemoglobin from Camphylobacter jejuni

Truncated hemoglobins (trHbs) have been shown to be loosely related in size and fold to myoglobin and monomeric hemoglobins. They have been located in unicellular prokaryotes and in some higher plants, where they play distinct roles. Based on sequence considerations, trHbs have been divided into three groups (I, II, and III); some microorganisms host trHbs from different groups, simultaneously, or at different times in their life span. Past work has shed light on the functional and structural properties of group I and II trHbs [1]. Here we shed first light on the structural features of group III trHbs. The 2.15 Å resolution crystal structure of trHbP from Camphylobacter jejuni shows that the 2-on-2 trHb fold is substantially conserved also in group III. The heme crevice presents important structural modifications in the C-D region and in the FG helical hinge. Contrary to what has been observed for group I and II trHbs, no protein matrix tunnel/cavity system appears evident in C. jejuni trHbP. Sequence conservation allows extrapolating part of the structural results here reported to the whole trHb group III.