Dissociation constants (Kd) for fluoride binding were measured as a function of pH for hemoglobin (Hb), myoglobin (Mb), horseradish peroxidase (HRP), and hemoglobins (HbI, HbII, and HbIII) from L. pectinata from pH 4.5 to 8.5. Our study shows that the affinity for fluoride is strongest at low pH for all the studied heme proteins. HbI from L. pectinata has the lowest affinity for fluoride binding with an average Kd of 0.1. This can be attributed to the weak interactions between the distal glutamine and the heme-bound fluoride. On the other hand, HbII and HbII have very strong affinity for fluoride with an average Kd value of 0.002. These strong interactions are the results of hydrogen bonding between the distal tyrosine and the heme-bound fluoride in both of these heme proteins. HRP has as strong affinity for fluoride binding as HbII and HbIII, however, the strong heme pocket interactions are generated by electrostatic interactions between the positively charged distal arginine and the heme-bound fluoride. In Hb and Mb, from pH 4 to pH 6, the dissociation constant for fluoride binding shows a clear pH dependence caused by the deprotonation of the distal histidine. From this study, the determined histidines pKas are 5.7 in the presence of heme-bound fluoride. In each case that was examined, it was evident how the distal amino acid impacted fluoride binding. The results of this study indicate the utility of fluoride ion as a probe of heme pocket-protein interactions.

Heme pocket interactions with fluoride in the hemoglobins from Lucina pectinata

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.