Hemoglobin Cooperativity Research Articles

The emergence of protein dynamics simulations: how computational statistical mechanics met biochemistry

In this essay, we aim to illustrate how Martin Karplus and his research group effectively set in motion the engine of molecular dynamics (MD) simulations of biomolecules. This process saw its prodromes between 1969 and the early 1970s with Karplus’ landing in biology, a transition that came to fruition with the treatment of 11-cis-retinal photoisomerization and the development of an allosteric model to account for the mechanism of cooperativity in hemoglobin. In 1977, J. Andrew McCammon, Bruce Gelin, and Martin Karplus published an article in Nature reporting the MD simulation of bovine pancreatic trypsin inhibitor (BPTI). This publication helped initiate the merger of computational statistical mechanics and biochemistry, a process that Karplus undertook at a later stage and whose beginnings we propose to reconstruct in this article through unpublished accounts of the key people who participated in this endeavor.

On the Origin of Hemoglobin Cooperativity under Non-equilibrium Conditions.

Abnormal hemoglobins can have major consequences for tissue delivery of oxygen. Correct diagnosis of hemoglobinopathies with altered oxygen affinity requires a determination of hemoglobin oxygen dissociation curve, which relates the hemoglobin oxygen saturation to the partial pressure of oxygen in the blood. Determination of the oxygen dissociation curve of human hemoglobin is typically carried out under conditions in which hemoglobin is in equilibrium with O2 at each partial pressure. However, in the human body due to the fast transit of red blood cells through tissues hemoglobin oxygen exchanges occur under non-equilibrium conditions. We describe the determination of non-equilibrium oxygen dissociation curve and show that under these conditions the true nature of hemoglobin cooperativity is revealed as emerging solely from the consecutive binding of oxygen to each one of the four subunits of hemoglobin until the entire tetramer is saturated. We call this form of cooperativity the sequential cooperativity of hemoglobin and define the simplest model that includes it as the minimalist model of hemoglobin. A single instantiation of this model accounts for ~70% of hemoglobin cooperativity under non-equilibrium conditions. The total cooperativity of hemoglobin can be viewed more correctly as the summation of two instantiations of the minimalist model (each one corresponding to a tetramer of low and high affinity for O2, respectively) in equilibrium with each other, as in the Monod-Wyman-Changeux model of hemoglobin. In addition to offering new insights on the nature of hemoglobin reaction with oxygen, the methodology described here for the determination of hemoglobin non-equilibrium oxygen dissociation curve provides a simple, fast, low-cost alternative to complex spectrophotometric methods, which is expected to be particularly valuable in regions where hemoglobinopathies are a significant public health problem, but where highly specialized laboratories capable of determining a traditional oxygen dissociation curve are not easily accessible.

Structural origin of cooperativity in human hemoglobin: a view from different roles of α and β subunits in the α2β2 tetramer.

This mini-review, mainly based on our resonance Raman studies on the structural origin of cooperative O2 binding in human adult hemoglobin (HbA), aims to answering why HbA is a tetramer consisting of two α and two β subunits. Here, we focus on the Fe-His bond, the sole coordination bond connecting heme to a globin. The Fe-His stretching frequencies reflect the O2 affinity and also the magnitude of strain imposed through globin by inter-subunit interactions, which is the origin of cooperativity. Cooperativity was first explained by Monod, Wyman, and Changeux, referred to as the MWC theory, but later explained by the two tertiary states (TTS) theory. Here, we related the higher-order structures of globin observed mainly by vibrational spectroscopy to the MWC theory. It became clear from the recent spectroscopic studies, X-ray crystallographic analysis, and mutagenesis experiments that the Fe-His bonds exhibit different roles between the α and β subunits. The absence of the Fe-His bond in the α subunit in some mutant and artificial Hbs inhibits T to R quaternary structural change upon O2 binding. However, its absence from the β subunit in mutant and artificial Hbs simply enhances the O2 affinity of the α subunit. Accordingly, the inter-subunit interactions between α and β subunits are nonsymmetric but substantial for HbA to perform cooperative O2 binding.

Ordered Motions in the Nitric-Oxide Dioxygenase Mechanism of Flavohemoglobin and Assorted Globins with Tightly Coupled Reductases.

Nitric-oxide dioxygenases (NODs) activate and combine O2 with NO to form nitrate. A variety of oxygen-binding hemoglobins with associated partner reductases or electron donors function as enzymatic NODs. Kinetic and structural investigations of the archetypal two-domain microbial flavohemoglobin-NOD have illuminated an allosteric mechanism that employs selective tunnels for O2 and NO, gates for NO and nitrate, transient O2 association with ferric heme, and an O2 and NO-triggered, ferric heme spin crossover-driven, motion-controlled, and dipole-regulated electron-transfer switch. The proposed mechanism facilitates radical-radical coupling of ferric-superoxide with NO to form nitrate while preventing suicidal ferrous-NO formation. Diverse globins display the structural and functional motifs necessary for a similar allosteric NOD mechanism. In silico docking simulations reveal monomeric erythrocyte hemoglobin alpha-chain and beta-chain intrinsically matched and tightly coupled with NADH-cytochrome b5 oxidoreductase and NADPH-cytochrome P450 oxidoreductase, respectively, forming membrane-bound flavohemoglobin-like mammalian NODs. The neuroprotective neuroglobin manifests a potential NOD role in a close-fitting ternary complex with membrane-bound NADH-cytochrome b5 oxidoreductase and cytochrome b5. Cytoglobin interfaces weakly with cytochrome b5 for O2 and NO-regulated electron-transfer and coupled NOD activity. The mechanistic model also provides insight into the evolution of O2binding cooperativity in hemoglobin and a basis for the discovery of allosteric NOD inhibitors.

Bohr equation and the lost allosteric Bohr effects in symmetry.

Bohr, Hasselbalch and Krogh demonstrated a group of sigmoid curves under various carbon dioxide contents in 1904. Hill fitted these curves in 1910 with Hill equation without the physical meanings of Hill coefficient and dissociation constant. In 1965, Monod-Wyman-Changeux model (MWC) popularized the word “allostery” with 81 words of symmetry to define an orthosteric nature of cooperativity in a single and symmetric sigmoid curve. Paradoxically the MWC model didn’t quantify the homotropic Hill coefficient and confusingly described the symmetry of sigmoid shapes with three allosteric variables. A heterotropic Bohr equation, by clarifying the biophysical symmetry in allostery, suggests the solution of allosteric coefficients with only one Bohr variable. We reveal that the mathematical need of a fictional monomer by MWC model justify a symmetric logistic curve with a parabolic kernel of dissociation constant to model the 1904 sigmoid curves. The logistic-derived Bohr equation and its half-saturated P50 equation successfully used the embedded P50 values in the 1904 sigmoidal curves to quantify their hyperbolic conformational shifts and Hill coefficients (n) pending for a century. Both are the logarithmic functions of carbon dioxide. This truly quantitative Bohr equation digitizes the allosteric regulation of the orthosteric affinity by precisely cloning the original group of dissociation/association curves published in 1904. The Bohr equation honestly suggests that nature should have chosen the allosteric Bohr effects to modify hemoglobin to cope with the swift dynamic of gas exchange. The discovery of the Bohr function in Bohr equation challenges the feasibility of the orthosteric cooperativity of hemoglobin.

Non-Allosteric Cooperativity in Hemoglobin.

Hemoglobin (Hb) is the prototypical example of a cooperative protein. Cooperativity of Hb is largely accounted for by the oxygen-linked allosteric interconversion between the T and R states/structures. Allostery is such a powerful explanation of Hb cooperativity that the possibility of cooperative events occurring within each allosteric conformation, in the absence of any quaternary structural change has usually been overlooked, and actually experiments specifically aimed at detecting nonallosteric cooperativity have usually failed to do so. However there are strong, but often neglected, theoretical reasons pointing to the presence of nonallosteric cooperativity under common experimental conditions, that have recently raised new interest and have been thoroughly re-investigated. Non-allosteric cooperativity within T state Hb has often been invoked to describe puzzling experimental data, either as an intrinsic property of the macromolecule or as a consequence of the binding of non-heme ligands. Few convincing pieces of evidence exist for the former hypothesis, whereas very strong proofs are available for effector-induced non-allosteric cooperativity in hemoglobin. Moreover, non-allosteric cooperativity in THb may explain some hitherto puzzling findings, e.g. the bi-exponential O2 release from THb observed by Q.H. Gibson in oxygen pulse experiments, the invariance of L4 found by K. Imai, the cooperative ligand binding by crystals of T state Hb Rotschild, and, possibly, the cooperativity observed in at least some mixed metal hybrid Hbs.

Interfacial water effect on cooperativity and signal communication in Scapharca dimeric hemoglobin.

Cooperativity is important in controlling the biological functions of allosteric proteins. Understanding the detailed mechanisms of cooperativity and allosteric regulation in such proteins is essential to understanding their function; however, the mechanism by which allosteric proteins undergo conformational transitions to aid the ligand escape process and its relevance to interfacial water molecules is not well understood. Here, we perform molecular dynamics simulations to examine these issues in Scapharca dimeric hemoglobin. The effects of interfacial water on dimeric motion, ligand escape probability, gate function, and cross-correlation are considered. The results reveal that interfacial water exhibits an unbalanced stress distribution in the interface region, leading to a bias helix bundle motion that not only can expedite the escape of the first ligand but also can increase the interval between the escape of both ligands. Correspondingly, the gate function follows the same time scale as the F-helix movement, and the gate opening is non-stochastic; moreover, the inconsistent motion between the gate parts resembles cooperative behavior. An explicit analysis of the intersubunit communication map provides at least 14 signal transduction pathways. Our results significantly aid in understanding the role of interfacial water in manipulating cooperativity and will lead to further applications involving molecular machines.

High oxygen affinity hemoglobins

High oxygen affinity hemoglobins are responsible for rare and heterogeneous autosomic dominant genetic diseases. They cause pure erythrocytosis, sometimes accountable for hyperviscosity and thrombosis, or hemolysis. Differential diagnoses must be first ruled out. The diagnosis is based on the identification of a decreased P50, and their possible characterization by cation exchange-high performance liquid chromatography and capillary electrophoresis. Finally, genetic studies of the responsible globin chain gene will confirm the mutation. The prognosis mainly relies on the P50 decrease rate and on the hemoglobin cooperativity impairment. Disease management should be personalized, and it should primarily depend on smoking cessation and physical activity. Phlebotomy and platelet aggregation inhibitors’ prescriptions can be discussed. There is no contraindication to flights, high-altitude conditions, or pregnancy. Nevertheless, blood donation must be prohibited.

Effector-Linked High-Frequency Thermal Fluctuations of Globin (Changes in Protein Dynamics) Regulate the Oxyge-Affinity and Cooperativity of Hemoglobin

Diatomic ligands such as O2, CO, and NO, which are dissociating from their binding sites (hemes) of myoglobin (Mb) and hemoglobin (Hb), are tentatively trapped at physical barriers of globin matrix (the “Caged” states) [1-5], before either [A] undergoing bimolecular dissociation to (deoxy state + free ligand) or [B] reverting back to re-bind to the hemes (geminate-recombination) [5]. Majorities of the un-bonded ligands take Route [B], while only small percentages of the un-bonded ligands proceed through Route [A] [5]. This causes substantial (>103 to 104-fold) increases in the apparent ligand-affinity for Mb and Hb, compared with that of free protoheme-N-base complexes [5]. It is well known that binding of heterotropic effectors reduces the O2-affinity of Hb (increases in P50), whereas binding of O2 does not change the P50 value of Hb. Effector-linked enhancements of high-frequency thermal fluctuations of globin [6] increase the transparency of globin matrix toward migrating diatomic ligands to enhance Route [A] and to diminish Route [B], resulted in substantial (up to >103-fold) decrease in the apparent O2-affinity (KT and KR) or (up to 100-fold increases in P50) without detectable changes in static crystallographic structures of Hb as well as the coordination/electronic structures of the hemes in T(deoxy)- and R(oxy)-Hb [5,7,8].

Effects of heme modification on oxygen affinity and cooperativity of human adult hemoglobin

We substituted strongly electron-withdrawing trifluoromethyl ( CF3) group(s) as heme side chain(s) of human adult hemoglobin (Hb) to achieve large alterations of the heme electronic structure, in order to elucidate the relationship between the oxygen ( O2) binding properties of Hb and the electronic properties of heme peripheral side chains. The obtained results were compared with those of similar studies performed on myoglobin (Mb), e.g. (Nishimura R, Matsumoto D, Shibata T, Yanagisawa S, Ogura T, Tai H, Matsuo T, Hirota S, Neya S, Suzuki A, and Yamamoto Y. Inorg. Chem. 2014; 53: 9156–9165). These two proteins shared the common feature of a decrease in O2affinity upon the CF3substitution(s). Using the P50value, which is the partial pressure of O2required for 50% oxygenation of a protein, and the equilibrium constant ( p Ka) of the "acid-alkaline transition" in the met form of a protein as measures of the O2affinity and the electron density of heme Fe atom of the protein, respectively, a linear p Ka- log (1/P50) relationship was demonstrated for the Hb and Mb systems. The native Hb, however, deviated from the p Ka- log (1/P50) relationship, while the native Mb followed it. These results highlighted the significance of the vinyl side chains of the heme cofactor in the functional control of Hb through tertiary and quaternary structural changes upon the oxygenation of the protein.

Blood oxygen affinity increases during digestion in the South American rattlesnake, Crotalus durissus terrificus

Digesting snakes experience massive increases in metabolism that can last for many days and are accompanied by adjustments in the oxygen transport cascade. Accordingly, we examined the oxygen-binding properties of the blood in the South American rattlesnake (Crotalus durissus terrificus) during fasting and 24 and 48h after the snakes have ingested a rodent meal corresponding to 15% (±2%) of its own body mass. In general, oxygen–hemoglobin (Hb–O2) affinity was significantly increased 24h post-feeding, and then returned toward fasting values within 48h post-feeding. Content of organic phosphates ([NTP] and [NTP]/[Hb]), hemoglobin cooperativity (Hill's n), and Bohr Effect (ΔlogP50/ΔpH) were not affected by feeding. The postprandial increase in Hb–O2 affinity in the South American rattlesnake can be almost entirely ascribed by the moderate alkaline tide that follows meal ingestion. In general, digesting snakes were able to regulate blood metabolites at quite constant levels (e.g., plasma osmolality, lactate, glucose, and total protein levels). The level of circulating lipids, however, was considerably increased, which may be related to their mobilization, since lipids are known to be incorporated by the enterocytes after snakes have fed. In conclusion, our results indicate that the exceptional metabolic increment exhibited by C. d. terrificus during meal digestion is entirely supported by the aerobic pathways and that among the attending cardiorespiratory adjustments, pulmonary Hb–O2 loading is likely improved due to the increment in blood O2 affinity.

New insight on biological interaction analysis: new nanocrystalline mixed metal oxide SPME fiber for GC-FID analysis of BTEX and its application in human hemoglobin-benzene interaction studies.

Nanocrystalline mixed metal oxides (MMO) of various metal cations were synthesized and were used for coating a piece of copper wire as a new high sensitive solid phase micro extraction (SPME) fiber in extraction and determination of BTEX compounds from the headspace of aqueous samples prior to GC-FID analysis. Under optimum extraction conditions, the proposed fiber exhibited low detection limits, and quantification limits, good reproducibility, simple and fast preparation method, high fiber capacity and high thermal and mechanical durability. These are some of the most important advantages of the new fiber. The proposed fiber was used for human hemoglobin upon interaction with benzene. Binding isotherm, Scatchard and Klotz logarithmic plots were constructed using HS-SPME-GC data, accurately. The obtained binding isotherm analyzed using Hill method. The Hill parameters have been obtained by calculating saturation parameter from the ratio of measured chromatographic peak areas in the presence and absence of hemoglobin. In this interaction, Hill coefficient and Hill constant determined as (nH = 6.14 and log KH = 6.47) respectively. These results reveal the cooperativity of hemoglobin upon interaction with benzene.

Reverse Engineering the Cooperative Machinery of Human Hemoglobin

Hemoglobin transports molecular oxygen from the lungs to all human tissues for cellular respiration. Its α2β2 tetrameric assembly undergoes cooperative binding and releasing of oxygen for superior efficiency and responsiveness. Over past decades, hundreds of hemoglobin structures were determined under a wide range of conditions for investigation of molecular mechanism of cooperativity. Based on a joint analysis of hemoglobin structures in the Protein Data Bank (Ren, companion article), here I present a reverse engineering approach to elucidate how two subunits within each dimer reciprocate identical motions that achieves intradimer cooperativity, how ligand-induced structural signals from two subunits are integrated to drive quaternary rotation, and how the structural environment at the oxygen binding sites alter their binding affinity. This mechanical model reveals the intricate design that achieves the cooperative mechanism and has previously been masked by inconsistent structural fluctuations. A number of competing theories on hemoglobin cooperativity and broader protein allostery are reconciled and unified.

Oxygen blood transport during experimental sepsis

The aim of this study was to highlight the link between induced hypothermia and increased survival duration as observed in the septic model developed by the laboratory. To reach this objective, survival duration and blood oxygen transport capacity were measured at two temperatures-38 °C (induced normothermia) and 34 °C (induced hypothermia)-in septic rats. A prospective, randomized, experimental animal study. University laboratory. Forty-four male Sprague-Dawley rats (median weight, 232 g; range, 200-303 g). After anesthesia and obtention of the temperature goal, sepsis was induced by cecal ligation and perforation. Sepsis induction led to death 5 hrs 11 mins ± 0 hr 36 mins after cecal ligation and perforation at 38 °C. At this temperature, significant changes in blood oxygen transport capacity were observed in septic rats; Hill number decreased from 2.36 ± 0.10 (baseline group) to 1.99 ± 0.17 (septic group) (p = .008) and oxygen-hemoglobin affinity decreased and P50 increased from 41.40 ± 2.4 Torr (baseline group) to 51.17 ± 14.07 Torr (septic group). Furthermore, in normothermia, a significant increase of creatinine and albumin plasmatic concentrations was observed 4 hrs after sepsis induction. Survival duration was significantly higher in induced hypothermia (7 hrs 22 mins ± 0 hr 12 mins at 34 °C) compared with induced normothermia. At 34 °C, no significant change in blood oxygen transport capacity was observed. In the same way, exposure to 34 °C induced no change in measured plasmatic parameters except an increase in albumin concentration in septic rats compared with the baseline group. Sepsis led to a decrease of both oxygen hemoglobin cooperativity and affinity at 38 °C. By contrast, no change in these parameters was observed when sepsis was induced during hypothermia. Taken together, these results could be interpreted in normothermia septic rats as an adaptive mechanism that could enhance the release of oxygen at the tissue level. Hypothermia by slowing down sepsis evolution could increase survival duration.

Nucleosome-mediated cooperativity between transcription factors

Cooperative binding of transcription factors (TFs) to promoters and other regulatory regions is essential for precise gene expression. The classical model of cooperativity requires direct interactions between TFs, thus constraining the arrangement of TF sites in regulatory regions. Recent genomic and functional studies, however, demonstrate a great deal of flexibility in such arrangements with variable distances, numbers of sites, and identities of TF sites located in cis-regulatory regions. Such flexibility is inconsistent with cooperativity by direct interactions between TFs. Here, we demonstrate that strong cooperativity among noninteracting TFs can be achieved by their competition with nucleosomes. We find that the mechanism of nucleosome-mediated cooperativity is analogous to cooperativity in another multimolecular complex: hemoglobin. This surprising analogy provides deep insights, with parallels between the heterotropic regulation of hemoglobin (e.g., the Bohr effect) and the roles of nucleosome-positioning sequences and chromatin modifications in gene expression. Nucleosome-mediated cooperativity is consistent with several experimental studies, is equally applicable to repressors and activators, allows substantial flexibility in and modularity of regulatory regions, and provides a rationale for a broad range of genomic and evolutionary observations. Striking parallels between cooperativity in hemoglobin and in transcriptional regulation point to a general mechanism that can be used in various biological systems.

A Biophysical Model Of Interactions Between Transcription Factors And Chromatin

Binding of transcription factors (TFs) to DNA is critical for triggering a cascade of events that lead to gene expression. The role of chromatin in this process is not considered by traditional biochemical models of protein-DNA interaction, or is limited to the passive DNA sequestration by the nucleosomes. Taking into account dynamic structure of chromatin is important for understanding transcription regulation in eukaryotes.Here we present a biophysical model of interactions between TFs and chromatinized DNA. The model takes into account dynamics of nucleosomes as well as other important features of eukaryotic regulatory regions such as the clustering TF binding sites, nucleosome-positioning DNA signals etc. Our model demonstrates that a wide range of biological phenomena can be explained by interactions between TFs and chromatin, and provides a quantitative description of the following processes:•cooperative binding and synergistic action of non-interacting TFs;•access of TFs to chromatinized DNA;•displacement of nucleosomes from regulatory regions;•rapid evolutionary changes in arrangement and membership of TF-binding sites in eukaryotic regulatory regions.Strikingly, we found that cooperative binding of TFs to chromatinized DNA is identical to the Monod-Wyman-Changeux model of allosteric cooperativity in hemoglobin, pointing at a general mechanism of cooperativity employed in a range of biological systems. This parallel allowed us to use classical results in biochemistry to gain deep insights into the mechanisms of gene regulation.

Nucleosome-mediated cooperativity between transcription factors

AbstractCooperative binding of transcription factors (TFs) to cis-regulatory regions (CRRs) is essential for precision of gene expression in development and other processes. The classical model of cooperativity requires direct interactions between TFs, thus constraining the arrangement of TFs sites in a CRR. On the contrary, genomic and functional studies demonstrate a great deal of flexibility in such arrangements with variable distances, numbers of sites, and identities of the involved TFs. Such flexibility is inconsistent with the cooperativity by direct interactions between TFs. Here we demonstrate that strong cooperativity among non-interacting TFs can be achieved by their competition with nucleosomes. We find that the mechanism of nucleosome-mediated cooperativity is mathematically identical to the Monod-Wyman-Changeux (MWC) model of cooperativity in hemoglobin. This surprising parallel provides deep insights, with parallels between heterotropic regulation of hemoglobin (e.g. Bohr effect) and roles of nucleosome-positioning sequences and chromatin modifications in gene regulation. Characterized mechanism is consistent with numerous experimental results, allows substantial flexibility in and modularity of CRRs, and provides a rationale for a broad range of genomic and evolutionary observations. Striking parallels between cooperativity in hemoglobin and in transcription regulation point at a new design principle that may be used in range of biological systems.

A quantum-chemical picture of hemoglobin affinity

Understanding the molecular mechanism of hemoglobin cooperativity remains an enduring challenge. Protein forces that control ligand affinity are not directly accessible by experiment. We demonstrate that computational quantum mechanics/molecular mechanics methods can provide reasonable values of ligand binding energies in Hb, and of their dependence on allostery. About 40% of the binding energy differences between the relaxed state and tense state quaternary structures result from strain induced in the heme and its ligands, especially in one of the pyrrole rings. The proximal histidine also contributes significantly, in particular, in the alpha-chains. The remaining energy difference resides in protein contacts, involving residues responsible for locking the quaternary changes. In the alpha-chains, the most important contacts involve the FG corner, at the "hinge" region of the alpha(1)beta(2) quaternary interface. The energy differences are spread more evenly among the beta-chain residues, suggesting greater flexibility for the beta- than for the alpha-chains along the quaternary transition. Despite this chain differentiation, the chains contribute equally to the relaxed substitute state energy difference. Thus, nature has evolved a symmetric response to the quaternary structure change, which is a requirement for maximum cooperativity, via different mechanisms for the two kinds of chains.

Negative cooperativity in Root-effect hemoglobins: role of heterogeneity

In some animals, the oxygen transport capacity of blood decreases when pH is lowered, yielding oxygen binding curves with Hill-coefficients smaller than unity. This so-called Root effect is observed in several fishes and is important for creating large oxygen partial pressures locally, for example in the swim bladder. While there is general agreement on the physiological advantages of this effect, its molecular basis remains ambiguous. Various studies show that isoforms of hemoglobins usually are present in the hemolymph, when the Root effect is observed. Here, we show that in such a case the mixture of these isoforms can exhibit apparent negative cooperativity, although each component taken separately can be described by the MWC model. In other cases, isolated isoforms exhibit true negative cooperativity. The well established MWC model describes many cooperative phenomena of enzymes and respiratory proteins but is not capable of describing negative cooperativity. In order to model negative cooperativity within a single molecular species a decoupling model might be employed, as pointed out previously. However, simulations show that it is not mandatory to have species with negative cooperativity, in order to obtain the binding curves typically seen for whole blood. These two aspects of the Root effect will be discussed on the basis of data from the literature.

Electronic reorganization: Origin of sigma trans promotion effect

Binding of two ligands trans to each other by some transition metal complexes may be cooperative [Khoroshun et al., Mol Phys 2002, 100, 523]. Several interesting consequent effects include (i) inverse relationship between bond strength and binding affinity; (ii) smaller coordination barriers to formation of weaker bonds; (iii) enhancement of Lewis acidity with increased number of ligands. We describe a simple model, sigma trans promotion effect (TPE), which considers electronic reorganization between two Lewis structures, and predicts the above-mentioned effects. The applied result of present study is the unified perspective on several facts of heme chemistry. Particularly, we reiterate an important but often overlooked notion, developed previously within the spin pairing model [Drago and Corden, Acc Chem Res 1980, 13, 353], that, in hemoproteins, the proximal histidine and the distal ligand such as O2 or CO cooperate in promoting electronic reorganization. As a result, depopulation of dz2 orbital upon ligand binding contributes to the phenomenon of hemoglobin cooperativity. The presented density functional (B3LYP) calculations on realistic models, the processes of carbon monoxide binding by Fe(II) porphyrins and dinitrogen binding by triamido/triamidoamine Mo(III) complexes, particularly the evaluation of the coordination barriers due to spin-state change by location of the minima on seams of crossing, support the TPE model predictions. From a broader theoretical perspective, the present study would hopefully stimulate the development of much needed frameworks and tools for facile comparisons of wave functions and their properties between different geometries, species, and electronic states. Advancement of practical wave function comparisons may yield fresh qualitative perspectives on chemical reactivity, and promote better understanding of related concepts such as electronic reorganization.