Hc1 Hc2

FIGURE 5-6 A comparison of the structures of myoglobin (PDB ID 1MBO) and the 3 subunit of hemoglobin (derived from PDB ID 1HGA).

FIGURE 5-7 The amino acid sequences of whale myoglobin and the a and fi 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 Hb«, 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«, and Hb^, respectively. Nonhelical residues at the amino and carboxyl termini, beyond the first (A) and last (H) «-helical segments, are labeled NA and HC, respectively.

FIGURE 5-8 Dominant interactions between hemoglobin subunits.

In this representation, a subunits are light and ft subunits are dark. The strongest subunit interactions (highlighted) occur between unlike subunits. When oxygen binds, the a1ft1 contact changes little, but there is a large change at the a1ft2 contact, with several ion pairs broken (PDB ID 1HGA).

FIGURE 5-8 Dominant interactions between hemoglobin subunits.

In this representation, a subunits are light and ft subunits are dark. The strongest subunit interactions (highlighted) occur between unlike subunits. When oxygen binds, the a1ft1 contact changes little, but there is a large change at the a1ft2 contact, with several ion pairs broken (PDB ID 1HGA).

of the ion pairs that stabilize the T state are broken and some new ones are formed.

Max Perutz proposed that the T n R transition is triggered by changes in the positions of key amino acid side chains surrounding the heme. In the T state, the porphyrin is slightly puckered, causing the heme iron to protrude somewhat on the proximal His (His F8) side. The binding of O2 causes the heme to assume a more planar conformation, shifting the position of the proximal His and the attached F helix (Fig. 5-11). These changes lead to adjustments in the ion pairs at the a1ft2 interface.

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