Zinc and magnesium substitution in hemoglobin: cyclic electron transfer within mixed-metal hybrids and crystal structure of MgHb

Studies of long-range electron transfer within mixed-metal hemoglobin (Hb) hybrids ( alpha sub(2)(FeP), beta sub(2)(MP)) (M = Mg, Zn; P = protoporphyrin IX) are reported, along with the X-ray crystal structure of magnesium-substituted hemoglobin (MgHb). MgHb adopts the quaternary structure of deoxyHb, and replacement of Fe by Mg causes negligible structural changes, supporting earlier inferences that electron transfer (ET) in these hybrids occurs between redox centers held at fixed and crystallographically known distance and orientation. Upon flash photolysis of the (MP, Fe super(3+)(H sub(2)O)P) hybrids, the charge-separated intermediate ((MP) super(+), Fe super(2+)P) (I) is formed by a photoinitiated super(3)(MP) arrow right Fe super(3+)P intramolecular electron-transfer process with rate constant k sub(t) and returns to the ground state by a Fe super(2+)P arrow right (MP) super(+) thermal electron transfer with rate constant k sub(b). By use of the transient absorption technique, we have measured k sub(t) and k sub(b) for M = Mg and Zn as a function of temperature between 0 and 25 degree C. The rate constant, k sub(1) = 35 (8)/s, for Mg at room temperature is significantly lower than that of Zn, k sub(t) = 85 (15)/s, although the driving force is greater in the former by about 100 mV.