INFLUENCE OF STERIC FACTORS ON OXYGEN BINDING .1. STUDIES ON 2,4-DIISOPROPYLDEUTEROHEME-MYOGLOBIN

Sperm whale apomyoglobin was recombined with 2,4-diisopropyldeuterohemin to form 2,4-diisopropyldeuteroheme-myoglobin, and its various physicochemical properties were investigated to get an insight into the structural and functional role of the peripheral vinyl groups. 2,4-Diisopropyldeuteroheme-myoglobin showed a 4 times lower O2 affinity at 25.degree. C and larger enthalpy and entropy changes of oxygenation than the corresponding values of native myoglobin. 2,4-Diisopropyldeuteroheme-metmyoglobin showed a pKa value of 9.68 which was higher than those of native metmyoglobin and mesoheme-metmyoglobin. The rate of autooxidation of oxy-form was about 7 times larger in 2,4-diisopropyldeuteroheme-myoglobin than in native myoglobin. The electron-donating effect of isopropyl groups did not give a straightforward explanation for these anomalous properties of 2,4-diisopropyldeuteroheme-myoglobin. Site and stereospecific van der Waals'' interaction between the polypeptide side chains and the peripheral 2,4-diisopropyl groups may weaken the interaction between the bound O2 molecule and the distal histidine, resulting in a decrease in the stability of oxy-form.