Locating the Binding Sites of Pb(II) Ion with Human and Bovine Serum Albumins

Lead is a potent environmental toxin that has accumulated above its natural level as a result of human activity. Pb cation shows major affinity towards protein complexation and it has been used as modulator of protein-membrane interactions. We located the binding sites of Pb(II) with human serum (HSA) and bovine serum albumins (BSA) at physiological conditions, using constant protein concentration and various Pb contents. FTIR, UV-visible, CD, fluorescence and X-ray photoelectron spectroscopic (XPS) methods were used to analyse Pb binding sites, the binding constant and the effect of metal ion complexation on HSA and BSA stability and conformations. Structural analysis showed that Pb binds strongly to HSA and BSA via hydrophilic contacts with overall binding constants of KPb-HSA = 8.2 (±0.8)×104 M−1 and KPb-BSA = 7.5 (±0.7)×104 M−1. The number of bound Pb cation per protein is 0.7 per HSA and BSA complexes. XPS located the binding sites of Pb cation with protein N and O atoms. Pb complexation alters protein conformation by a major reduction of α-helix from 57% (free HSA) to 48% (metal-complex) and 63% (free BSA) to 52% (metal-complex) inducing a partial protein destabilization

Inhibition of the water oxidizing complex of photosystem II and the reoxidation of the quinone acceptor QA- by Pb2+.

The action of the environmental toxic Pb(2+) on photosynthetic electron transport was studied in thylakoid membranes isolated from spinach leaves. Fluorescence and thermoluminescence techniques were performed in order to determine the mode of Pb(2+) action in photosystem II (PSII). The invariance of fluorescence characteristics of chlorophyll a (Chl a) and magnesium tetraphenylporphyrin (MgTPP), a molecule structurally analogous to Chl a, in the presence of Pb(2+) confirms that Pb cation does not interact directly with chlorophyll molecules in PSII. The results show that Pb interacts with the water oxidation complex thus perturbing charge recombination between the quinone acceptors of PSII and the S2 state of the Mn4Ca cluster. Electron transfer between the quinone acceptors QA and QB is also greatly retarded in the presence of Pb(2+). This is proposed to be owing to a transmembrane modification of the acceptor side of the photosystem

Fluorescence studies of retinol and retinyl acetate

The polarized emission study of all-trans retinol and retinyl acetate in solid films at room temperature has been carried out. The shape of the absorption and fluorescence is typical of Franck-Condon forbidden bands. The high positive polarization of the fluorescence excitation and the emission (when excited into main absorption band due to 1 Bu ← 1 Ag transition) suggests that the lowest excited singlet state of retinols has a large amount of 1 Bu character. The explanation for the high positive polarization involving strong vibronic interactions suggesting 1 Ag - (a very weakly allowed state) being the lowest excited singlet has also been considered

Polarized fluorescence study of retinals at room temperature

The solid films of all-trans, 9-cis, and 13-cis retinals have been found to be fluorescent at room temperature. Their polarization study with the aim of understanding the nature of their fluorescent state has also been carried out at room temperature. The high positive polarization of fluorescence with respect to the excitation carried out in the main 1Bu ← 1Ag absorption transition of retinals suggests that their fluorescent state possesses a large amount of 1Bu character. A vibronic coupling mechanism that suggests the 1Ag− (a weakly allowed π,π*) state or 1n,π* state being the lowest singlet state in retinals has also been considered

Monolayer Studies of Visual Pigments

One of the most interesting and fascinating facts about biology is the existence of ordered structures in the organelles. The electron microscopic and X-ray diffraction studies have shown that in living systems the molecules are, in fact, arranged in strict order and are present in aggregates