生物大分子结构与功能第5章蛋白质的柔性结构.ppt

The structure of myoglobin Oxygen binds to heme with the O2 axis at an angle, a binding conformation readily accommodated by myoglobin. (b) Carbon monoxide binds to free heme with the CO axis perpendicular to the plane of the porphyrin ring. When binding to the heme in myoglobin, CO is forced to adopt a slight angle because the perpendicular arrangement is sterically blocked by His E7, the distal His. This effect weakens the binding of CO to myoglobin. (c) Another view (derived from PDB ID 1MBO), showing the arrangement of key amino acid residues around the heme of myoglobin. The bound O2 is hydrogen-bonded to the distal His, His E7 (His64), further facilitating the binding of O2. Dynamics of oxygen release by myoglobin The bending of O2 to the heme in myoglobin depends on molecular motion, or “breath” in protein structure. One major route is provided By rotation of the side chain of distal His (His64). The rate-limiting process in oxygen release is the opening of a pathway for the O2 molecule to escape from the heme pocket. Oxygen may spend time rattling in its cage - and perhaps being recaptured - before the tertiary structure of the myoglobin shifts enough to let it escape Hemoglobin Hemoglobin (Mr 64,500) is a tetrameric protein Containing four heme groups, one associated with each polypeptide chain. Adult hemoglobin contains Two types of globin, two c chains (141 residues) and two β chains (146 residues). Fewer than half of the amino acide residues in poly- Peptide sequences of α and β subunits are identical, Their three-dimensional structures are very similar, And also similar to that of myoglobin In a multisubunit protein, a conformational change in one subunit often affects the conformation of other subunits. Interactions between ligands and proteins may be regulated, usually through specific interactions with one or more additional ligands. These other ligands may cause conformational changes in the protein that affect th