ExbB and ExbD are cytoplasmic membrane proteins that associate with TonB to convey the energy of the proton-motive force to outer membrane receptors in Gram-negative bacteria for iron uptake. The opportunistic pathogen Serratia marcescens (Sm) possesses both TonB and a heme-specific TonB paralog, HasB. ExbB _Sm has a long periplasmic extension absent in other bacteria such as E. coli (Ec). Long ExbB's are found in several genera of Alphaproteobacteria, most often in correlation with a hasB gene. We investigated specificity determinants of ExbB _Sm and HasB. We determined the cryo-EM structures of ExbB _Sm and of the ExbB-ExbD _Sm complex from S. marcescens. ExbB _Sm alone is a stable pentamer, and its complex includes two ExbD monomers. We showed that ExbB _Sm extension interacts with HasB and is involved in heme acquisition and we identified key residues in the membrane domain of ExbB _Sm and ExbB _Ec , essential for function and likely involved in the interaction with TonB/HasB. Our results shed light on the class of inner membrane energy machinery formed by ExbB, ExbD and HasB.