by: Deepika Satish
Abstract
Studying the evolution of globins and observing patterns of oxygen migration is one method of determining how nature has selectively chosen properties for oxygen storage and transport. In this project, we have studied Scapharca inaequivalvis Hemoglobin I (ScHbI), a dimeric protein found in Scapharca inaequivalvis clams, to map its internal migration of oxygen and compare the observed pathway to the distal histidine ‘gate’ pathway found in both monomeric mammalian myoglobins and tetrameric adult human hemoglobin. For ScHbI, this putative distal histidine gate (His-69(E7)) appears to be closed by the hemoglobin’s inverted quaternary structure; thus, we tested if it was the ligand migration pathway in ScHbI by first changing the size and polarity at and around the distal histidine gate and then examining the effects on oxygen association, dissociation, and affinity. Our results strongly suggest that, despite facing the dimer interface, ScHbI’s E7 residue functions as the gate for diatomic ligand capture in the distal pocket, thus having a similar oxygen migration pathway as mammalian myoglobins and human hemoglobins. By studying ScHbI and other model hemoglobins, we are building up a set of rules that govern the function of all hemoglobins. These rules can then be employed to optimize the properties of recombinant human hemoglobin and to engineer a hemoglobin-based oxygen carrier or blood substitute for the treatment of hemorrhagic shock due to severe blood loss.
Blood Substitutes
As the demand for whole donated blood increases every day, the need for blood substitutes becomes more and more important for the treatment of hemorrhagic shock under emergency situations. Over the years, much effort has been expended on making safe, effective blood substitutes, but none have been able to perfectly emulate human hemoglobin (HbA) within intact red blood cells. A hemoglobinbased oxygen carrier (HBOC) named PolyHeme® is one of the few oxygen-carrying blood substitutes to complete a phase III trial. However, it failed to receive FDA approval in April 2009 because the studies revealed that “Polyheme® places the patients at a higher risk of significant adverse events”,1 such as heart attacks and myocardial infarctions.2 Another hemoglobin-based blood substitute known as Hemopure ® is the first and only oxygen-carrying blood substitute to be approved for human use in South Africa. However, in the United States, analyses of the phase III human trials conducted using Hemopure® were brought into question, and in mid-2002, the FDA mandated that further animal trials on nitric oxide scavenging and blood pressure effects be conducted before any new human clinical trials could resume.3,4 This brings to view just one of the many obstacles faced when developing blood substitutes. When hemoglobin- based blood substitutes have low oxygen association, they tend to bind nitric oxide (known as NO scavenging), which triggers vasoconstriction. This results in hypertension, a major adverse effect of current hemoglobin-based blood substitutes.
Dr. John S. Olson’s laboratory has focused on engineering recombinant hemoglobin (Hb) that has fewer blood pressure side effects and increased oxygen delivery efficiency. His laboratory has conducted multiple molecular biophysics studies exploring the optimization of the properties required for a successful blood substitute. Furthermore, recombinant hemoglobins from multiple organisms are being studied to understand how their unique structures affect their oxygen binding properties and to apply this knowledge in developing strategies for engineering extracellular hemoglobin as a blood substitute.
Engineering Recombinant Hemoglobin
Designing recombinant hemoglobin for use as a blood substitute necessitates the creation of a mutant with moderately low oxygen affinity, large oxygen exchange rates, high subunit cooperativity, and minimum rates of nitric oxide scavenging.5 In order to synthetically optimize the function of recombinant hemoglobin in attaining these properties, we require a general understanding of how globins evolved in nature under selective pressure for cooperative and rapid oxygen binding and release. A general knowledge base will enable us to develop a set of rules governing the speed of ligand (oxygen and carbon monoxide) migration into the protein and the stereochemistry of iron-ligand bond formation within the globin active site. With a progressively better understanding of oxygen migration paths within these proteins, we will be able to understand how gases migrate within proteins in general and to identify patterns of pathways for different families of hemoglobins.6 Most importantly, we will be able to predict oxygen pathways in various types of hemoglobin and modify them by protein engineering. Although many different types of oxygen pathways in globins have been mapped thus far, the best understood is sperm whale myoglobin (SwMb). Its oxygen interactions have been studied extensively, and it has served as the model system of protein dynamics for many years. 7,8 While SwMb studies have helped shed light on the basics of oxygen migration, a key feature of the globins that has not been extensively explored is if or how cooperative interactions between subunits alter ligand pathways. Multimers, such as HbA, often display cooperativity through inter-monomer conformational changes. 9,10 In this study, hemoglobin from the mollusk Scapharca inaequivalvis (ScHbI) was chosen as the model system because it represents the simplest allosteric system of two identical subunits in which one subunit alters the other’s ligand affinity at a chemically identical active site and because the interface appears to block the distal histidine gate found in mammalian Mbs and Hbs. 11
Scapharca inaequivalvis Hb I
Mammalian hemoglobin is a complex tetrameric protein that delivers oxygen to and transports carbon dioxide away from various tissues in the body (Figure 1A). The protein has four subunits, each containing a prosthetic group known as heme. Each heme molecule contains one iron that binds to a single diatomic oxygen molecule. In SwMb and HbA, ligands gain access to this heme located in the ligand binding pocket by moving through a channel created by upward and outward movement of the distal histidine E7 side chain. This mechanism for ligand entry is known as the His(E7) gate. ScHbI is thought to have a His(E7) gate similar to that in Mb and HbA.8,12 However, the His(E7) gates of this dimeric hemoglobin appear to be closed by its inverted quaternary structure, as each subunit seems to cover and block the outward movement its adjacent subunit’s His(E7) gate. Thus, to bind to the heme within the distal pocket, oxygen would have to first migrate into the dimer interface, and then proceed past the His(E7) gate (Figure 1B),13 unlike in HbA, which has ‘open’ (E7) gates that are not blocked by the molecule’s own subunits (Figure 1A). Thus, whether oxygen still enters and exits ScHbI through the E7 gate is an open question.7
In this project, mutagenesis has been conducted at five key helical positions (E7, CD3, CD4, F4, and B3), which are either at the E7 gate or within the ligand pocket, to create the library of ScHbI mutants, most of which are shown in Figure 3. These mutations were intended to demonstrate a wide range of effects, all of which were chosen to determine if the E7 gate has a role in ligand migration. If oxygen enters through the E7 gate, then varying the size and polarity of the amino acids at the putative gate (E7) should show large effects on the rate of ligand association; altering the ease of opening of the gate and the amount of internal water that must be displaced before oxygen can bind would result in measureable changes in oxygen association and dissociation.14 Altering amino acids within the distal pocket (at positions CD3 and CD4) should affect the ease of movement of the His(E7) gate and thus the rate of migration of ligands into and out of the distal pocket. Lastly, mutations to reduce cooperativity (at position F4) and to render a monomeric form of the protein (at position B3) should simplify the model such that the role of the E7 gate can be further elucidated.15,16,17 The recombinant hemoglobin protein was constructed based on the protocol prescribed by Summerford et al.18 By observing the effects of these changes in the oxygen binding rate and equilibrium constants of the hemoglobin and identifying the trends that depend on the size and flexibility of the side chain at the E7 position, we can determine whether the distal histidine gate is the route of ligand migration in ScHbI.
Mapping Ligand Migration
Time courses for O2 and CO binding were measured and used to determine ligand association (k’) and dissociation (k) rates and equilibrium association constants (K). By looking at trends in the oxygen rate and affinity constants for the library of ScHbI mutants in terms of the size and polarity of the altered side chains, we can map the pathway of oxygen migration and examine the strength of electrostatic and steric interactions with the bound ligand.
In these kinetic experiments, CO is photodissociated from the heme group of ScHbI in the presence of O2 by a flash tube-driven dye laser, which provides an intense light pulse for ~0.5 μs at 577 nm. When the absorbance changes in the sample are followed at 436 nm, we can observe the formation of deoxyHb (hemoglobin without any bound ligand) during the laser excitation pulse, and then follow its decay as ligands rebind from solvent in a bimolecular process. The initial fast bimolecular rebinding phase (kf) represents rebinding of O2 and some CO at the rate given in the equations below. If the absorbance changes are followed at 425 nm, a slow unimolecular phase (ks) is observed and represents the displacement of oxygen that was bound in the fast phase by carbon monoxide, which has a greater affinity for the protein. Expressions for the rate of this slow phase and the overall oxygen affinity constant, which is calculated from the association (k’O2) and dissociation (kO2) constants, are also given below:
Sets of k’O2, kO2, and KO2 were obtained for the different mutants, after which trends and correlations with increasing size or polarity were examined.
Analysis of the kinetic data for ScHbI suggests that the major pathway for ligand entry and exit in ScHbI is through the E7 gate despite the gate’s apparent closure by the adjacent subunit. In general, increasing the size of the amino acid at the E7 position decreases the rate of oxygen association up to 10-fold, indicating the involvement of the E7 gate in oxygen migration. This is further supported by the fact that changing the polarity of the E7 amino acid has a great effect on the rates of both ligand association and dissociation. Polar amino acids that are capable of donating a hydrogen bond markedly stabilize bound oxygen. However, these same amino acids stabilize water in the binding pocket of deoxyScHbI and slow the rate of ligand association. Thus, when histidine, asparagine, and glutamine are present at the E7 position, there is a complex tradeoff between the effects of size and polarity of the amino acid on oxygen association and dissociation rates. Mutations at the CD4, CD3, F4, and B3 positions also indicate that the main path of ligand migration is through the dimer interface and the E7 gate.
In general, the pattern of mutagenesis effects observed for ScHbI resembles those reported previously for the subunits of HbA and SwMb, which suggests that the distal histidine moves outward to allow diatomic ligand entry and exit.12,8. Thus, our work with ScHbI indicates that the basic rules governing the ligand migration and affinity in ScHbI are identical to those for mammalian Hbs and Mbs and apply in general to all globins with a distal histidine.
Future Work
While the experiments done so far on ScHbI all indicate that the E7 gate is the route of ligand migration, a few more mutants need to be made to fully demonstrate the range of effects that the mutations have on the rate of oxygen association and dissociation. This project has enabled us to further understand the evolution of globins and helped us to elucidate a model on which future experiments can be based, but much work still needs to be done to completely understand animal hemoglobins and establish principles for globin design. Properties required for a hemoglobin-based blood substitute include moderately low oxygen affinity, high oxygen exchange rates, high subunit cooperativity, discrimination against CO binding, low rates of nitrogen oxide reaction, resistance to autooxidation, resistance to reactions of H2O2, resistance to denaturation, and high affinity to heme,5 of which only the first three properties were studied following the methods used in this project. However, all of these key ideas must undergo more extensive investigation, especially in HbA. Continuing research of hemoglobins of other organisms is also crucial to understanding the patterns for all of these properties throughout evolution.
Deepika Satish has worked in Dr. John S. Olson’s lab where she has constructed recombinant ScHbI, performed kinetic tests on the recombinant protein, and analyzed the trends of the kinetic tests in relation to the HbA. Previous such hemoglobin research conducted in the Olson lab has yielded ideas that have led to the construction of safer second-generation hemoglobin-based blood substitutes by the Baxter Hemoglobin Therapeutics.5 Continued research in this area will hopefully contribute more elucidations and ideas for more efficient and effective blood substitute construction. Deepika is a sophomore Biochemistry and Cell Biology major at Will Rice College ’12.