Aluminum (III) Interactions with Sulfur-Containing Amino Acid Chains

Positive selection in octopus haemocyanin indicates functional links to temperature adaptation

BACKGROUND

Octopods have successfully colonised the world's oceans from the tropics to the poles. Yet, successful persistence in these habitats has required adaptations of their advanced physiological apparatus to compensate impaired oxygen supply. Their oxygen transporter haemocyanin plays a major role in cold tolerance and accordingly has undergone functional modifications to sustain oxygen release at sub-zero temperatures. However, it remains unknown how molecular properties evolved to explain the observed functional adaptations. We thus aimed to assess whether natural selection affected molecular and structural properties of haemocyanin that explains temperature adaptation in octopods.

RESULTS

Analysis of 239 partial sequences of the haemocyanin functional units (FU) f and g of 28 octopod species of polar, temperate, subtropical and tropical origin revealed natural selection was acting primarily on charge properties of surface residues. Polar octopods contained haemocyanins with higher net surface charge due to decreased glutamic acid content and higher numbers of basic amino acids. Within the analysed partial sequences, positive selection was present at site 2545, positioned between the active copper binding centre and the FU g surface. At this site, methionine was the dominant amino acid in polar octopods and leucine was dominant in tropical octopods. Sites directly involved in oxygen binding or quaternary interactions were highly conserved within the analysed sequence.

CONCLUSIONS

This study has provided the first insight into molecular and structural mechanisms that have enabled octopods to sustain oxygen supply from polar to tropical conditions. Our findings imply modulation of oxygen binding via charge-charge interaction at the protein surface, which stabilize quaternary interactions among functional units to reduce detrimental effects of high pH on venous oxygen release. Of the observed partial haemocyanin sequence, residue 2545 formed a close link between the FU g surface and the active centre, suggesting a role as allosteric binding site. The prevalence of methionine at this site in polar octopods, implies regulation of oxygen affinity via increased sensitivity to allosteric metal binding. High sequence conservation of sites directly involved in oxygen binding indicates that functional modifications of octopod haemocyanin rather occur via more subtle mechanisms, as observed in this study.

Aluminium in biological environments: a computational approach

The increased availability of aluminium in biological environments, due to human intervention in the last century, raises concerns on the effects that this so far “excluded from biology” metal might have on living organisms. Consequently, the bioinorganic chemistry of aluminium has emerged as a very active field of research. This review will focus on our contributions to this field, based on computational studies that can yield an understanding of the aluminum biochemistry at a molecular level. Aluminium can interact and be stabilized in biological environments by complexing with both low molecular mass chelants and high molecular mass peptides. The speciation of the metal is, nonetheless, dictated by the hydrolytic species dominant in each case and which vary according to the pH condition of the medium. In blood, citrate and serum transferrin are identified as the main low molecular mass and high molecular mass molecules interacting with aluminium. The complexation of aluminium to citrate and the subsequent changes exerted on the deprotonation pathways of its tritable groups will be discussed along with the mechanisms for the intake and release of aluminium in serum transferrin at two pH conditions, physiological neutral and endosomatic acidic. Aluminium can substitute other metals, in particular magnesium, in protein buried sites and trigger conformational disorder and alteration of the protonation states of the protein's sidechains. A detailed account of the interaction of aluminium with proteic sidechains will be given. Finally, it will be described how alumnium can exert oxidative stress by stabilizing superoxide radicals either as mononuclear aluminium or clustered in boehmite. The possibility of promotion of Fenton reaction, and production of hydroxyl radicals will also be discussed.

Complexation of Al(III) with reduced glutathione in acidic aqueous solutions

The complexation of reduced glutathione (GSH) in its free and Al(III)-bound species in acidic aqueous solutions was characterized by means of multi-analytical techniques: pH-potentiometry, multinuclear ((1)H, (13)C and (27)Al) and two-dimensional nuclear Overhauser enhancement NMR spectroscopy ((1)H, (1)H-NOESY), electrospray mass spectroscopy (ESI-MS), and ab initio electronic structure calculations. The following results were found. In the 25 degrees C 0.1M KCl and 37 degrees C 0.15M NaCl ionic medium systems, Al(3+) coordinates with the important biomolecule GSH through carboxylate groups to form various mononuclear 1:1 (AlHL, AlH(2)L and AlH(-1)L), 1:2 (AlL(2)) complexes, and dinuclear (Al(2)H(5)L(2)) species, where H(4)L(+) denotes totally protonated GSH. Besides the monodentate complexes through carboxylate groups, the amino groups and the peptide bond imino and carbonyl groups may also be involved in binding with Al(3+) in the bidentate and tridentate complexes. The present data reinforce that the glycine carboxylate group of GSH has a higher microscopic complex formation constant than gamma-glutamyl carboxylate. Compared with simple amino acids, the tripeptide GSH displays a greater affinity for the Al(3+) ion and thus may interfere with aluminum's biological role more significantly.

Complexation of AlIII by Aromatic Amino Acids in the Gas Phase

The coordination properties of three natural aromatic amino acids (AAAs)-phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp)-to AlIII are studied in this work, devoting special attention to the role of the aromatic side chain. A comparison with aluminum(III)-alanine complexes is also presented. The polarizability arising from the ring has been seen to be a key factor in the stability of the complexes, with the order being Trp-AlIII > Tyr-AlIII > Phe-AlIII, starting from the most stable one. Cation-pi interactions between the metal and the aromatic ring are present in the lowest energy conformers, especially for Trp, which seems to be very well suited for these kinds of interactions, occurring with both the six- and five-membered rings of the indole side chain. The most stable coordination mode for the three AAAs is found to be tricoordinated with the N and O of the backbone chain and the aromatic ring, as was found theoretically and experimentally for other metals.