Calcium binding site(s) of Photosystem II as probed by lanthanides

Terbium Excitation Spectroscopy as a Detection Method for Chromatographic Separation of Lanthanide-Binding Biomolecules

Studies of biosorption and bioaccumulation of heavy metals deal mostly with challenging, inhomogeneous, and complex materials. Therefore, most reports describe only application studies, while fundamental research is limited to indirect methods and speculations on the binding mechanisms. In this study, we describe a method for detecting and isolating heavy metal-binding biomolecules directly from crude extracts. The underlying principle is terbium sensitization and fluorescence excitation spectroscopy used offline after a chromatographic run. Compounds interacting with metal ions inevitably change the coordination sphere of terbium, which is reflected in the excitation spectrum leading to metal-specific luminescence. Main advantages of our approach include simple, fast, and inexpensive experiment design, nondestructive measurements, and detection limits far below 1 mg. Here, we have applied our method for three promising biosorbents (green algae, moss, and cyanobacterium) and obtained first information on the character of active compounds isolated from each species.

Use of lanthanides to alleviate the effects of metal ion-deficiency in Desmodesmus quadricauda (Sphaeropleales, Chlorophyta)

Lanthanides are biologically non-essential elements with wide applications in technology and industry. Their concentration as environmental contaminants is, therefore, increasing. Although non-essential, lanthanides have been proposed (and even used) to produce beneficial effects in plants, even though their mechanisms of action are unclear. Recently, it was suggested that they may replace essential elements. We tested the effect of low concentrations of lanthanides on the common freshwater microalga Desmodesmus quadricauda, grown under conditions of metal ion-deficiency (lower calcium or manganese concentrations). Our goal was to test if lanthanides can replace essential metals in their functions. Physiological stress was recorded by studying growth and photosynthetic activity using a pulse amplitude modulation (PAM) fluorimeter. We found that nutrient stress reduced parameters of growth and photosynthesis, such as maximal quantum yield, relative electron transport rate, photon capturing efficiency and light saturation irradiance. After adding low concentrations of five lanthanides, we confirmed that they can produce a stimulatory effect on microalgae, depending on the nutrient (metal) deprivation. In the case of a calcium deficit, the addition of lanthanides partly alleviated the adverse effects, probably by a partial substitution of the element. In contrast, with manganese deprivation (and at even lower concentrations), lanthanides enhanced the deleterious effect on cellular growth and photosynthetic competence. These results show that lanthanides can replace essential elements, but their effects on microalgae depend on stress and the nutritional state of the microalgae, raising the possibility of environmental impacts at even low concentrations.

Accessibility of tyrosine Y(.)(Z) to exogenous reductants and Mn(2+) in various Photosystem II preparations

The reduction of tyrosine Y(.)(Z) by benzidine and exogenous Mn(2+) was studied by kinetic EPR experiments in various Photosystem II (PSII) preparations. Using lanthanide treated PSII membranes it was demonstrated that neither the extrinsic polypeptides (17, 23 and 33 kDa) nor the Mn complex block the accessibility of Y(.)(Z) to exogenous reductants, such as benzidine. In addition, it was shown that in the presence of the native Mn complex exogenous Mn(2+) does not reduce Y(.)(Z).

Recent advances in the understanding of the biological chemistry of manganese

Developments in manganese biochemistry have centered on the discovery of new manganese enzymes, X-ray analysis of binuclear manganese enzymes, and the discovery of new spectroscopic signatures for the oxygen-evolving complex. Despite these gains, many questions regarding the structure, composition and redox state of the oxygen-evolving complex remain unanswered.

Near-infrared Yb(3+) vibronic sideband spectroscopy: application to Ca(2+)-binding proteins

We have used near-infrared (NIR) vibronic fluorescence spectroscopy to study the vibrational structure of ligands associated with model complexes of the lanthanide Yb(3+). This technique exploits the similar binding properties of the lanthanide Yb(3+) to probe Ca(2+)-binding sites in proteins. The (NIR) fluorescence of complexed Yb(3+) exhibits, in addition to main 0-0 (2F5/2----2F7/2) electronic transition of Yb(3+), weak vibronic sidebands which provide infrared-like, local vibrational spectra of the chelates (inner sphere ligands) of Yb(3+). A similar approach has been used for the lanthanide Gd(3+) (MacGregor, R.B., Jr (1989) Arch. Biochem. Biophys. 274, 312-316) which fluoresces in the UV and which is usually complicated by amino-acid residues fluorescing in the same spectral region. In this same spectral region, other complications in studying photosynthetic membranes occur in the form of the excitation wavelength being actinic, promoting photodegradation of the membranes, as well as the reabsorption of Gd(3+) fluorescence. NIR excitation and fluorescence detection of Yb(3+) avoid these problems when studying photosynthetic membranes. A preliminary study has been conducted here on rat muscle parvalbumin.