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Abstract

Heme oxygenase catalyzes the first step in the oxidative degradation of heme. The crystal structure of heme oxygenase-1 (HO-1) reported here reveals a novel helical fold with the heme sandwiched between two helices. The proximal helix provides a heme iron ligand, His 25. Conserved glycines in the distal helix near the oxygen binding site allow close contact between the helix backbone and heme in addition to providing flexibility for substrate binding and product release. Regioselective oxygenation of the alpha-meso heme carbon is due primarily to steric influence of the distal helix.

In silico analysis of heme oxygenase structural homologues identifies group‐specific conservations

“…Four of five residues are involved in hydrophobic packing, as observed in the hHO1 structure 30. Leu17{132} is 69% conserved overall and was leucine in thiaminases, RNR1As, and HOs except plant species.…”

“…A G143H mutation inactivated both mouse and human HO1 28, 29. Tyr58{184} is part of a larger hydrogen bonding network, which will be discussed below 30. Note that the degree of conservation in the entire alignment, highlighted in Fig.…”

“…Gly40{166} is 63% conserved overall and is found in thiaminases and HOs except plant species. This residue is found in the turn between α‐1 and α‐2 30. Conserved glycines are often found in turns in enzyme structures or at positions where a side chain would lead to steric conflicts in many enzyme families 31, 32, 33, 34.…”

“…Tyr114{268} is 66% conserved overall and is found in all HOs and most thiaminases except proteins identified as Tregs. This residue is found in α‐4 in hHO1 30. In hHO1, the side chain hydroxyl of Tyr114{268} is 3.1 Å from NH1 of Arg136{327} (not conserved) from the distal helix of HOs, and 3.0 Å from ND1 of Asn210{423}.…”

Two epitopes responsible for the catalytic activity of heme oxygenase‐1 identified by phage display

“…The rat HO‐1 (rHO‐1) is 32 kDa in size and is composed of 289 amino acid residues. The crystal of HO‐1 with heme demonstrates that heme is sandwiched between a proximal A‐helix (Leu13‐Glu29) and a distal F‐helix (Leu129‐Met155) of HO‐1 consisting of eight α‐helices (A–H), where the His25 serves as the proximal ligand and Gly139 and Gly143 are close to the distal ligand of the heme iron 7, 8, 9. Conserved Gly143 was shown to provide the flexibility required for the opening and closure of heme activity with respect to substrate binding and product dissociation during HO‐1 catalysis 10, 11.…”