Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Hemoglobin Subunits Are Structurally Similar to Myoglobin

Hemoglobin (Mr 64,500; abbreviated Hb) is roughly spherical, with a diameter of nearly 5.5 nm. It is a tetrameric protein containing four heme prosthetic groups, one associated with each polypeptide chain. Adult hemoglobin contains two α types of globin, two chains (141 residues each) and two β chains (146 residues each). Although fewer than half of the amino acid residues in the polypeptide sequences of the α and β subunits are identical, the three-dimensional structures of the two types of subunits are very similar. Furthermore, their structures are very similar to that of myoglobin (Fig. 5–6), even though the amino acid sequences of the three polypeptides are identical at only 27 positions (Fig. 5–7). All three polypeptides are mem bers of the globin family of proteins. The helix-naming convention described for myoglobin is also applied to the hemoglobin polypeptides, except that the subunit lacks the short D helix. The heme-binding pocket is made up largely of the E and F helices.

FIGURE 5–6 A comparison of the structures of myoglobin (PDB ID 1MBO) and the subunit of hemoglobin (derived from PDB ID 1HGA).

FIGURE 5–7 The amino acid sequences of whale myoglobin α and β the and chains of human hemoglobin. Dashed lines mark helix boundaries. To align the sequences optimally, short gaps must be introduced into both Hb sequences where a few amino acids are present in the compared sequences. With the exception of the missing D helix in Hbα, this alignment permits the use of the helix lettering convention that emphasizes the common positioning of amino acid residues that are identical in all three structures (shaded). Residues shaded in pink are conserved in all known globins. Note that the common helix-letter and-number designation for amino acids does not necessarily correspond to a common position in the linear sequence of amino acids in the polypeptides. For example, the distal His residue is His E7 in all three structures, but corresponds to His⁶⁴, His⁵⁸, and His⁶³ in the linear sequences of Mb, Hb α, and Hb β, respectively. Nonhelical residues at the amino and carboxyl termini, beyond the first (A) and last (H) α -helical segments, are labeled NA and HC, respectively.

FIGURE 5–8 Dominant interactions between hemoglobin subunits. In this representation, α subunits are light and β subunits are dark. The strongest subunit interactions (highlighted) occur between unlike subunits. When oxygen binds, the α₁ β₁ contact changes little, but there is a large change at the α₁ β₂ contact, with several ion pairs bro ken (PDB ID 1HGA).

The quaternary structure of hemoglobin features strong interactions between unlike subunits. The α₁ β₁ interface (and its α₂β₂ counterpart) involves more than 30 residues, and its interaction is sufficiently strong that although mild treatment of hemoglobin with urea tends to cause the tetramer to disassemble into αβ dimers, these dimers remain intact. The α₁β₂ (and α₂β₁) inter face involves 19 residues (Fig. 5–8). Hydrophobic in teractions predominate at the interfaces, but there are also many hydrogen bonds and a few ion pairs (sometimes referred to as salt bridges), whose importance is discussed below.

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مواضيع ذات صلة

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Cooperative Ligand Binding Can Be Described

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Hemoglobin Undergoes a Structural Change on Binding Oxygen

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Oxygen Is Transported in Blood by Hemoglobin

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Protein Structure Affects How Ligands Bind

Membrane Dynamics:- Transbilayer Movement of Lipids Requires Catalysis

The Composition and Architecture of Membranes:- Lipids and Proteins Diffuse Laterally in the Bilayer

Membrane Dynamics:- Acyl Groups in the Bilayer Interior Are Ordered to Varying Degrees

The Composition and Architecture of Membranes:- Covalently Attached Lipids Anchor Some Membrane Proteins

The Composition and Architecture of Membranes:- The Topology of an Integral Membrane Protein Can Be Predicted from Its Sequence

The Composition and Architecture of Membranes: -Integral Proteins Are Held in the Membrane by Hydrophobic Interactions with Lipids

The Composition and Architecture of Membranes:- All Biological Membranes Share Some Fundamental Properties

The Composition and Architecture of Membranes:- Each Type of Membrane Has Characteristic Lipids and Proteins