The T-to-R transformation in hemoglobin: a reevaluation.

The relationship between the T, R, and R2 quaternary forms of hemoglobin is examined by computational experiments. Contrary to previous suggestions, we propose that the R quaternary form may lie on the pathway from T to R2. This proposal is consistent with four independent observations. (i) Difference distance maps are used to identify those parts of the molecule that undergo conformational change upon oxygenation. The simplest interpretation of these maps brackets R between T and R2. (ii) Linear interpolation from T to R2 passes through R. (iii) The well-known "switch" region (so called because, upon transition between the T and R quaternary forms, a residue from the beta 2 subunit toggles between two stable positions within the alpha 1 subunit) progresses from T through R to R2, successively. (iv) A hitherto-undocumented feature, diagnostic of the R structure, is noted within the alpha subunit: upon transformation from T to R, the beta-turns at the amino terminal of the E and F helices flip from one turn type to another. Upon transformation from R to R2, the latter turn--a strained conformation--flips back again.