Heme oxygenase isoforms

All mammalian cells are virtually capable of producing CO with heme as the main substrate (Fig. 1) [5]. Enzymatic heme metabolism in vivo is mainly catalyzed by heme oxygenase (HO). In the presence of HO, the porphyrin ring of heme is broken and oxidized at the a-methene bridge, producing equimolar amounts of CO, ferrous iron, and biliverdin. Three isoforms of HO have been identified. Inducible HO-1 (32 kDa) is mostly recognized for its upregulation in response... 

Heme oxygenase catalyzes the first and rate limiting step in the degradation of heme to give biliverdin (Fig. 15). Humans and other mammals have two HO isoforms termed HO-1 and HO-2. Human HO-1 consists of 288 residues (757), whereas rat HO-2 has 316 residues (752). HO contains a C-terminal membrane anchoring sequence (753) that is... 

Truncated forms of human and rat heme oxygenase-1 have been successfully crystallized (53, 89), and the structure of the human isoform has been determined (Fig. 6) (53). The full-length human heme oxygenase is a 288-residue protein, but the protein that was crystallized... 

The inducible isoforms of the enzymes cyclooxygenase (COX 2), nitric oxide synthase (iNOS) and heme oxygenase 1 (HO-1) have generated great interest as possible therapeutic targets in inflammation. This book is the first publication to address the importance of all three enzymes and the consequences of their interactions to the inflammatory process. 

In 1989, BH4 was found to be a cofactor for nitric oxide synthase (NOS) [ 126, 127]. BH4 is also involved in dimerization of NOS, as NOS is catalytically active in a homodimer structure. Three isoforms of NOS exist neuronal NOS (NOS 1), inducible NOS (NOS 2) and endothelial NOS (NOS 3). BH4 is essential for all NOS isoforms. The NOS isoforms share approximately 50-60% sequence homology. Each NOS polypeptide is comprised of oxygenase and reductase domains. An N-terminal oxygenase domain contains iron protoporphyrin IX (heme), BH4 and an arginine binding site, and a C-terminal reductase domain contains flavin mononucleotide (FMN), and a reduced nicotin-amide adenine dinucleotide phosphate (NADPH) binding site. 

Three isoforms of NOS are produced in mammalian cells neuronal (nNOS), endothelial (eNOS), and inducible (iNOS) [55]. All NOS isoforms exist as homodimers with a C-terminal FMN-FAD fused reductase domain, an N-terminal oxygenase domain, and a calmodulin binding sequence at the interface of the two domains. The NOS catalytic mechanism is complicated and requires O2, NADPH, FMN, FAD, Ca2+, calmodulin, tetrahydro-biopterin (BH4), and heme to effect the five-electron oxidation of L-arginine to L-citrulline and NO. Consumed in this process are 1.5 equivalents of NADPH and 2 equivalents of O2. 

Dimerization of the oxygenase domains is essential for catalytic activity and for binding of the pterin cofactor [99]. Recent structural data for the dimeric oxygenase domains of NOS [100-102] reveal an extensive dimer interface that creates binding sites for the two pterins, sequesters the heme from the solvent, and helps to structure the substrate binding site (Figure 11). Upon dimerization, all three isoforms pro-... 

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