Comparison of ligand migration and binding in heme proteins of the globin family

The binding of small diatomic ligands such as carbon monoxide or dioxygen to heme proteins is among the simplest biological processes known. Still, it has taken many decades to understand the mechanistic aspects of this process in full detail. Here, we compare ligand binding in three heme proteins of the globin family, myoglobin, a dimeric hemoglobin, and neuroglobin. The combination of structural, spectroscopic, and kinetic experiments over many years by many laboratories has revealed common properties of globins and a clear mechanistic picture of ligand binding at the molecular level. In addition to the ligand binding site at the heme iron, a primary ligand docking site exists that ensures efficient ligand binding to and release from the heme iron. Additional, secondary docking sites can greatly facilitate ligand escape after its dissociation from the heme. Although there is only indirect evidence at present, a preformed histidine gate appears to exist that allows ligand entry to and exit from the active site. The importance of these features can be assessed by studies involving modified proteins（via site-directed mutagenesis） and comparison with heme proteins not belonging to the globin family.

Highly Luminescent Positively Charged Quantum Dots Interacting with Proteins and Cells

We have studied interactions between positively charged MUTAB-stabilized quantum dots(QDs)and model proteins,serum and live cells using fluorescence correlation spectroscopy(FCS),dynamic light scattering(DLS),time-resolved photoluminescence(PL)and live-cell fluorescence imaging.Using human serum albumin(HSA)as a model protein,we measured the growth of a protein adsorption layer(“protein corona”)via time-resolved FCS.Corona formation was characterized by an apparent equilibrium dissociation coefficient,KD≈10μM.HSA adlayer growth was surprisingly slow(timescale ca.30 min),in stark contrast to many similar measurements with HSA and other proteins and different NPs.Time-resolved PL data revealed a characteristic quenching behavior depending on the QD surface coverage with HSA.Taken together,we found that MUTAB-QDs initially bind HSA molecules weakly(KD≈700μM);however,the affinity is enhanced over time,presumably due to proton injection into the MUTAB layer by HSA triggering ligand dissociation.This process was also observed with human blood serum,showing equal kinetics for comparable HSA concentration.Moreover,imaging experiments with cultured human cells(HeLa)revealed that MUTAB-QDs bind to the cell membrane and perforate it.This process is reduced upon pre-adsorption of proteins on the MUTAB-QD.