Oxygen-Binding Proteins: Structure, Function, and Kinetics

Physiological Challenges of Oxygen Transport

  • Oxygen (O2) Solubility and Diffusion:

    • Oxygen is poorly soluble in aqueous solutions.

    • Diffusion of oxygen through tissues is inefficient over long distances.

    • Larger, multicellular organisms require specialized mechanisms for transporting and storing O2 to meet metabolic demands.

  • Requirements for Transport Mechanisms:

    • The interaction between the transporter and O2 must be highly specific.

    • The binding must be reversible to allow for both loading in lungs/gills and unloading in tissues.

The Role of Transition Metals and Heme

  • Metal Complexes:

    • Oxygen is complexed with transition metals because metals like iron (FeFe) and copper (CuCu) have high affinities for O2O_2.

    • Free iron is problematic in biological systems because it can promote the formation of highly reactive oxygen species (ROS).

    • To mitigate this tendency, iron is incorporated into a prosthetic group called heme.

  • Structure of Heme:

    • Heme is defined as a complex of Protoporphyrin IX and Fe2+Fe^{2+}.

    • Porphyrin Component:

      • Consists of four pyrrole rings.

      • The rings are connected by methine (-CH=\text{-CH=}) bridges.

      • The structure is linked into a conjugated C=CC=C double bond system.

      • Substitutions at the "X" positions on the rings define the specific type of porphyrin. In heme, two of the "X" groups are propionate groups.

      • There are four nitrogen (NN) atoms in the center that can bind a metal ion.

  • Iron Oxidation States:

    • Ferrous (Fe2+Fe^{2+}): This state binds O2O_2 reversibly.

    • Ferric (Fe3+Fe^{3+}): This state does not bind O2O_2.

    • In heme-containing proteins, the irreversible oxidation of Fe2+Fe^{2+} to Fe3+Fe^{3+} is prevented by burying the heme within the protein structure.

Classification and Structure of Globin Proteins

  • Types of Globins:

    • Myoglobin (Mb): A monomeric protein that binds and stores O2O_2 in muscle tissue.

    • Hemoglobin (Hb): A tetramer consisting of two α\alpha-globins and two β\beta-globins; it serves as the primary O2O_2 transporter in the blood.

    • Leghemoglobin: Found in leguminous plants; it sequesters O2O_2 to protect O2O_2-sensitive enzymes in nitrogen (N2N_2)-fixing bacteria.

  • General Structure of Globins:

    • Globins are globular α\alpha-helical proteins.

    • They consist of eight α\alpha-helices, denoted letters A through H (from the N-terminus to the C-terminus).

    • Naming Conventions:

      • Helices: A, B, C, D, E, F, G, H.

      • Termini: There is no "N" helix; the letter "N" represents the Amino terminus. There is a "C" helix, but the letter "C" is also used for the Carboxyl terminus. To distinguish, the C-terminus is labeled "HC".

      • Loops: Connecting loops are named after the two helices they join (e.g., AB, CD, DE, FG).

      • Residues: Amino acids are identified by their relative position within a structural motif.

        • Example: CD1 is the first amino acid in the loop between helices C and D.

        • Example: F8 is the 8th amino acid in helix F.

        • Example: NA2 is the second residue between the N-terminus and helix A.

        • Example: HC2 is the second residue between helix H and the C-terminus.

  • Physical Properties of Myoglobin (Mb):

    • Composed of a single polypeptide chain.

    • Contains 153 amino acid residues.

    • Molecular weight is approximately 16.7 kDa16.7\,kDa.

The Heme Binding Pocket and Coordination

  • Microenvironment:

    • The heme binding pocket is primarily formed by the E and F helices.

    • Propionate side chains of the heme are positioned near the surface of the globin.

    • The remainder of the heme is surrounded by nonpolar residues, with the exception of two critical histidines.

  • Coordination Bonds of Iron:

    • The iron atom in heme has six coordination positions.

    • Four coordination bonds are occupied by the nitrogens of the porphyrin ring.

    • The 5th Coordination Bond (Proximal Histidine): Occupied by the side-chain nitrogen of a Histidine residue at position F8 (His93His^{93} in myoglobin, His87His^{87} in α\alpha-hemoglobin).

    • The 6th Coordination Bond: This is the site where O2O_2 binds reversibly.

  • The Distal Histidine (E7):

    • Located at position E7 (His64His^{64} in Myoglobin).

    • It is close to the heme but not directly bonded to the iron.

    • It is positioned on the side where O2O_2 binds and interacts with the bound O2O_2 molecule.

Quantifying Ligand Binding

  • Basic Definitions and Constants:

    • [P][P] = concentration of free protein.

    • [L][L] = concentration of free ligand.

    • [PL][PL] = concentration of ligand-bound protein.

    • Association Constant (KaK_a): Measures the affinity of the protein for the ligand.

      • P+L⇌PLP + L \rightleftharpoons PL

      • Ka=[PL][P][L]K_a = \frac{[PL]}{[P][L]}

    • Dissociation Constant (KdK_d): The reciprocal of KaK_a, representing the concentration of ligand at which half of the binding sites are occupied.

      • Kd=1Ka=[P][L][PL]K_d = \frac{1}{K_a} = \frac{[P][L]}{[PL]}

  • The Binding Equation (Occupancy):

    • The fraction of binding sites occupied, denoted by θ\theta (theta) or YY, is defined as:

      • θ=binding sites occupiedtotal binding sites\theta = \frac{\text{binding sites occupied}}{\text{total binding sites}}

      • θ=[PL][PL]+[P]\theta = \frac{[PL]}{[PL] + [P]}

    • By substitution, θ\theta can be expressed in terms of ligand concentration:

      • θ=[L][L]+Kd\theta = \frac{[L]}{[L] + K_d}

    • For non-cooperative binding, the plot of θ\theta vs. [L][L] is a hyperbolic function.

  • Constants in Biological Systems:

    • High affinity corresponds to a low KdK_d value.

    • Low affinity corresponds to a high KdK_d value.

    • Examples of KdK_d values:

      • Biotin-Avidin: 1×10−15 M1 \times 10^{-15}\,M (nearly irreversible).

      • Insulin Receptor: 1×10−10 M1 \times 10^{-10}\,M.

      • Anti-HIV Immunoglobulin (gp41): 4×10−10 M4 \times 10^{-10}\,M.

      • Nickel-binding protein: 1×10−7 M1 \times 10^{-7}\,M.

      • Calmodulin (Ca2+): 3×10−6 M3 \times 10^{-6}\,M.

      • Enzyme-substrate interactions: Typically range from 10−310^{-3} to 10−7 M10^{-7}\,M.

  • Oxygen Binding Specifics:

    • For gases, concentration is replaced by partial pressure (pO2pO_2).

    • θ=pO2pO2+P50\theta = \frac{pO_2}{pO_2 + P_{50}}

    • P50P_{50} is the partial pressure of O2O_2 at which 50% of the sites are saturated.

    • Myoglobin binds O2O_2 with high affinity (P50P_{50} is low). In tissues, where pO2pO_2 is approximately 4 kPa4\,kPa, myoglobin is essentially saturated and insensitive to pressure changes.

Optical Properties and Protein Flexibility

  • Spectral Changes:

    • The conjugated double bond system of heme absorbs visible light.

    • Oxygen binding shifts the electron distribution, changing the light absorption spectrum.

    • Oxy-heme: Absorbs more blue light, allowing more red light to pass (appears bright red).

    • Deoxy-heme: Absorbs more red light, making it appear bluish.

    • This difference distinguishes arterial (oxygenated) blood from venous (deoxygenated) blood and allows for experimental measurement of O2O_2 binding.

  • Protein "Breathing":

    • Binding depends on protein flexibility.

    • Small molecular motions of side chains (< 1 A˚1\,Å) occur on a nanosecond time scale, allowing ligands to enter and exit the binding pocket.

Specificity and Carbon Monoxide (CO) Inhibition

  • Binding Affinities for Carbon Monoxide:

    • Carbon monoxide (CO) binds to free heme with 20,00020,000 times greater affinity than O2O_2.

    • In the context of Hemoglobin (Hb), CO's affinity is only about 250250 times that of O2O_2.

    • In Myoglobin (Mb), CO's affinity is approximately 4040 times that of O2O_2.

  • Structural Basis for Selectivity:

    • Linear vs. Angled Binding: CO naturally binds to heme in an upright, linear geometry. O2O_2 binds to heme at an angle.

    • Steric Hindrance: The Distal Histidine (E7) creates steric hindrance that forces CO out of its preferred linear orientation, lowering its binding affinity.

    • Hydrogen Bonding: The Distal Histidine (E7) forms a favorable hydrogen bond with the bound O2O_2 molecule, which is not possible with CO. This stabilizes the O2O_2 binding.

  • Clinical Relevance:

    • The selectivity provided by the protein structure protects the organism from low levels of CO.

    • Atmospheric CO is roughly 0.1 ppm0.1\,ppm, leading to a θO2\theta_{O2} of 0.9990.999.

    • Slightly elevated CO (10 ppm10\,ppm) results in a θO2\theta_{O2} of 0.980.98.

    • Without the protein environment, free heme would be nearly 50% saturated by CO even at low concentrations (θO2∼0.5\theta_{O2} \sim 0.5), which would be fatal.