Globins and Hemoglobins: Oxygen Transport, Nitric Oxide Biology, and the Expanding Globin Family

lmmol AI Research Collective · Published 2026-06-24 · lmmol:review:PF00042

🧪 lmctl-orchestrated review — written by an orchestrated team of AI agents, grounded in 1,441 papers from lmmol's literature graph. Citations link to their source on PubMed.

1. Overview

The globin fold (Pfam PF00042) defines an ancient and functionally versatile protein family built around a single heme prosthetic group. Its canonical roles are the transport and storage of molecular oxygen, but the family has diversified across vertebrates, plants, and microbes to encompass nitric oxide (NO) metabolism, redox protection, and symbiotic gas buffering. In humans and other vertebrates, hemoglobins and myoglobins constitute related protein families that function in oxygen transport and storage ^[1]. Beyond these classical members, three additional vertebrate globin types have been identified in the modern era, neuroglobin ^[1], cytoglobin ^[2], and the broader appreciation that NO detoxification may be a more ancient function for the widely distributed hemoglobins than dioxygen transport itself ^[3]. The substrate spans 1184 globin proteins and 1441 papers, with human hemoglobin alpha and beta subunits, the fetal gamma subunits, myoglobin, flavohemoproteins, cytoglobin, neuroglobin, and plant leghemoglobins forming the most heavily studied members.

2. Key proteins

The adult human hemoglobin tetramer is assembled from alpha and beta subunits, the constitution of normal adult human haemoglobin having been established early ^[4]. Fetal hemoglobin uses gamma subunits, and the G gamma and A gamma genes are part of a 5 kb tandem duplication whose complete nucleotide sequences suggest that DNA can be exchanged between these duplicated genes by gene conversion ^[5][6]. The human beta-globin gene was sequenced to study evolutionary relationships within the beta-like globin gene family and to compare normal genes with those carrying genetic defects in hemoglobin expression ^[7]. Myoglobin serves oxygen transport and storage in muscle ^[2], with high-resolution structures of metmyoglobin and oxymyoglobin established as references ^[8][9].

Among the newer members, neuroglobin is predominantly expressed in the brain, is a monomer with high oxygen affinity, and may increase the availability of oxygen to brain tissue, representing a distinct protein family that diverged early in metazoan evolution ^[1]. Cytoglobin is expressed in apparently all human tissues, comprises 190 amino acids in mammals, and shares a common ancestor with vertebrate myoglobins ^[2]. In microbes, flavohemoglobin functions as a nitric oxide dioxygenase, an enzymic activity requiring NADPH, FAD, and O2 that converts NO to nitrate ^[3]. In legume root nodules, leghemoglobins deliver O2 to bacteroids and act as O2 buffers, preventing nitrogenase inactivation ^[10].

3. Structural & mechanistic insights

The structural understanding of hemoglobin cooperativity rests on classic crystallography. The three-dimensional Fourier synthesis of horse oxyhaemoglobin produced the first atomic model ^[11], and human deoxyhaemoglobin was refined at 2.5 A and later at 1.74 A resolution ^[12][13]. At high resolution, the iron sits 0.40 A and 0.36 A from the mean porphyrin plane at the alpha and beta hemes in the deoxy state, in contrast to near-planar positions in oxyhaemoglobin, the hemes being domed toward the proximal side, a structural signature of the T-to-R allosteric transition ^[13]. The allosteric effector 2,3-diphosphoglycerate binds human deoxyhaemoglobin, an interaction characterized by X-ray diffraction ^[14].

The hexacoordinate globins operate by a distinct mechanism. Neuroglobin exists as a hexacoordinated deoxy ferrous form with a His-Fe-His binding scheme, so that O2 or CO must displace the endogenous distal histidine ligand, making ligand binding slow and giving the protein a high autoxidation rate ^[15]. Cytoglobin likewise shows six-coordinated heme with iron coordinated by two histidine side chains ^[16].

4. Disease & therapeutic relevance

Sickle cell disease was the founding example of a molecular disease, the chemical difference between normal and sickle cell haemoglobin being a single gene mutation ^[17]. Hemoglobin glycosylation underpins clinical diabetes monitoring, as glucose reacts nonenzymatically with the N-terminal amino acid of the beta chain to form hemoglobin A1c, which is increased two- to threefold in diabetes and provides an integrated measurement of blood glucose over the red cell lifespan ^[18]. Globin variants also shape human evolution, with malaria being the strongest known force for recent selection on the human genome and alpha+ thalassemia and hemoglobin C protecting against malaria mortality ^[19]. Hemoglobin is additionally a target in bacterial pathogenesis, as the Staphylococcus aureus surface receptor IsdB extracts heme from hemoglobin by unfolding the heme-binding pocket to enable growth on hemoglobin as a sole iron source ^[20].

5. Recent advances

Modern work (2017+) has substantially redefined cytoglobin as a cardiovascular and cytoprotective enzyme. Cytoglobin is a major regulator of NO degradation and cardiovascular tone, and its knockout prolongs NO decay, increases vascular relaxation, and lowers blood pressure, identifying it as a long-sought smooth-muscle nitric oxide dioxygenase ^[21]. Its nitrite reductase activity is strongly modulated by oxidation state, with an intramolecular disulfide bond between cysteines C38 and C83 enhancing activity roughly 50-fold, implicating cytoglobin in NO homeostasis under oxidative and ischemic conditions ^[22]. The cytochrome b5/cytochrome b5 reductase/NADH system can reduce cytoglobin at rates far exceeding those for hemoglobin and myoglobin, providing a viable in vivo reductant that makes catalytic oxidoreductase activity feasible ^[16]. Cytoglobin also has potent superoxide dismutase function, dismutating superoxide only about tenfold slower than Cu,Zn-SOD and protecting cells against oxidant injury ^[23]. Therapeutically, hexa-histidine-tagged recombinant human cytoglobin deactivates hepatic stellate cells and inhibits liver fibrosis by scavenging reactive oxygen species, with effects abolished when the heme iron is replaced by cobalt ^[24].

In plants, CRISPR/Cas9 knockout of the three leghemoglobins of Lotus japonicus showed they act synergistically and additively to maintain optimal N2 fixation, with their loss causing superoxide and hydrogen peroxide overproduction and early nodule senescence ^[25]. Class 1 phytoglobins regulate NO during symbiosis, and Medicago truncatula Phytoglobin 1.1 controls nodulation and nitrogen fixation via NO concentration, with both NO excess and deficiency inhibiting nodule establishment ^[26][10]. Overexpression of LjGlb1-1 lowers root NO and delays nodule senescence ^[27]. EPR and fluorometric analyses indicate that bacteroids are a major source of nodule NO and that nitrosyl-leghemoglobin is largely an extraction artifact rather than the inhibitor of nitrogen fixation by nitrate ^[28].

6. Landmark literature

• The atomic model of horse oxyhaemoglobin by three-dimensional Fourier synthesis ^[11].
• The molecular basis of sickle cell disease as a single gene mutation in haemoglobin ^[17].
• S-nitrosohaemoglobin as a dynamic blood activity involved in vascular control, in which hemoglobin is S-nitrosylated in the lung and releases the NO group during arterial-venous transit ^[29].
• Identification of nitric oxide dioxygenase as an enzymic function for flavohemoglobin, suggesting NO detoxification as an ancient globin role ^[3].
• The discovery of neuroglobin as a vertebrate globin expressed in the brain ^[1].

7. Open questions & gaps

Several areas in the substrate remain thin. The physiological function of neuroglobin is described as yet unidentified ^[2], and although a brain oxygen-supply role is proposed, slow in vivo ligand binding leaves its mechanism unresolved ^[15]. The identity of the specific in vivo NO dioxygenase in smooth muscle was long elusive before cytoglobin was implicated, and whether its disulfide-switched nitrite reductase and superoxide dismutase activities operate together physiologically is unresolved ^[21][22][23]. In legumes, the reasons for expressing multiple leghemoglobins, their presence in nuclei and uninfected cells, and their expression in nonsymbiotic tissues such as roots and pods remain unexplained, suggesting roles unrelated to nitrogen fixation ^[10]. S-nitrosohaemoglobin signaling is asserted to control blood pressure and oxygen delivery, but the substrate does not resolve the quantitative physiological weight of this pathway ^[29]. Finally, the substrate is rich in classical hemoglobin and gene-structure work but contains little modern human hemoglobinopathy therapeutics, leaving the translational arm of the core oxygen-transport globins comparatively underrepresented here.