associations between dimers and also contribute to the cohesion of the tetrameric unit. Furthermore, electrostatic interactions between charged amino acid side chains and hydrogen-bonding interactions at the AB/CD dimer interface serve to stabilize the tetrameric structure. The COOH-terminal a-helices, which contain four lysine residues, are arranged as antiparallel pairs on the surface of each extended b-sheet (St. Charles et al., 1989). The lysine residues are predominantly on one side, resulting in a "ring of positive charge" that runs perpendicularly across the helices (Stuckey et al., 1992; Zhang et al., 1994) (Figs. 2 and 4; see also Chapter 6).

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