Metallomics And Metallomes

A schematic sketch of metallomics is proposed in Figure 15.1. On the left-hand side, academic technical terms such as genomics, proteomics, and metabollomics are shown along with metallomics to indicate their research areas in the biological system. Genomics deals with the scientific works on the genetic information of DNAs and RNAs encoded as the sequences of nucleic bases. DNA and RNA play an essential role in protein synthesis. Proteins are distributed inside and outside the cell, and they work as enzymes for synthesis and metabolism of various biomolecules of the cell. It is seen that a large number of proteins play essential roles in syntheses and metabolisms of many biological molecules to regulate and maintain the life system; protein science has been receiving great attention as postgenome science linked with genomics.

Many biological substances as well as metal ions are transported as raw materials inside the cell through the membrane. In general, material conversion is actively occurring inside the cell and also often in the cell membrane, and such material conversion and transportation in evolving specific transporters is termed "metabolism." Biological substances, which are usually small molecules such as amino acids, organic acids, and metal ions produced in metabolism, have recently been called "metabollomes" or "metabolites." Bioscience concerned with metallic elements and their applications has been studied independently in many scientific fields such as biochemistry; bioinorganic chemistry; nutritional science; pharmacy; medicine; toxicology; agriculture; and environmental science.

All such scientific fields have a deep interrelationship, with the common factor of metals, from the viewpoint of biological science. Therefore, it is desirable to promote it as an interdisciplinary field. Thus, Haraguchi [1] proposed the nomenclature of "metallomics" for biometal science. In the study of metallomics, elucidation of the physiological roles and functions of biomolecules binding with metallic ions in the biological systems should be the most important research target.

In recent years, genomics and proteomics have received great attention to appreciate various biological systems from the viewpoints of gene and protein sciences. Genomics and proteomics are indeed fundamentally important scientific fields because genes (DNAs and RNAs) contain the genetic information codes to synthesize various proteins. Genes and proteins cannot be synthesized without the assistance of metalloenzymes containing zinc and other metals. In this sense, metallomics may stand in the same position in scientific significance as genomics and proteomics. Thus, in metallomics, biological molecules bound with biometals are properly defined as "metallomes," corresponding to genomes and proteomes in genomics and proteomics, respectively. However, metallic ions such as alkali and alkaline earth metal ions, which exist mostly as free ions in biological fluids, should also be included in metallomes because they play many important roles in the occurrence of the physiological functions in the biological systems.

15.3 GLUTATHIONE AND ORGANIC ACIDS METABOLISM

Glutathione and organic acids metabolism plays a key role in metal tolerance in plants [2-5]. Glutathione is ubiquitous component cells from bacteria to plants and animals. In plants, it is the major low molecular mass thiol compound (28). Glutathione occurs in plants mainly as reduced GSH (95 %). Its synthesis is mediated by the enzymes glutamylcysteine synthetase (EC 6.3.2.2) and glutathione synthetase (EC 6.3.2.3). Glutathione metabolism is also connected with cysteine and sulphur metabolism in plants. Cysteine concentration limits glutathione biosynthesis. Low molecular thiol peptide phytochelatins (PCs), often called class III metallothioneins, are synthesized in plants from glutathione induced by heavy metal ions [6].

These peptides are synthesized from glutathione by means of a-glutamylcysteine transferase enzyme (EC 2.3.2.15), which is also called phytochelatin synthase (PCS), catalyzing transfer reaction of (a-Glu-Cys) group from a glutathione donor molecule to glutathione, an acceptor molecule. PCS is a cytosolic, constitutive enzyme and is activated by metal ions, namely, Cd2+, Pb2+, Ag1+, Bi3+, Zn2+, Cu2+, Hg2+, and Au2+. PCs thus synthesized chelate heavy metals and form complexes that are transported through cytosol in an ATP-dependent manner through tonoplast into vacuole. Thus, the toxic metals are swept away from cytosol. Some high molecular weight complexes (HMW) with S-2 can also be formed from these LMW complexes in vacuole [7].

Transgenic plants with modified genes of PCS and genes of glutathione synthesis enzymes, a-GCS and GS, and enzymes connected with sulphur metabolism, e.g., serineacetyltransferase, need special attention in order to achieve success in phytoremediation of metals in the environment. Plants under heavy metal stress produce free radicals and reactive oxygen species and must withstand the oxidative stress before acquiring tolerance to toxic metals. Glutathione is then used for the synthesis of PCs as well as for dithiol (GSSG) production. The ascorbate-glutathione pathway is involved in plant defense against oxidative stress. Organic acids play a major role in metal tolerance [8].

Organic acids play a role in metal chelation by forming complexes with metals, a process of metal detoxification. Chelation of metals with exuded organic acids in the rhizosphere and rhizospheric processes indeed form an important aspect of investigation for remediation. These metabolic pathways underscore the physiological, biochemical, and molecular bases for heavy metal tolerance [6].

15.4 METAL TRANSPORTERS AND INTERACTIONS IN MEMBRANES AT MOLECULAR LEVELS

Plants and humans require adequate amounts of micronutrients like iron and zinc, but accumulation of an excess or uptake of nonessential metals like cadmium or lead can be extremely harmful.

Proteins of the CDF (cation diffusion facilitator) family are involved in the homeostasis of Cd2+, Co2+, Fe2+, and Zn2+ in microbes, animals, and plants [9]. Therefore, elucidation of the role of CDF proteins in Arabidopsis thaliana would be advantageous to the success of phytoremediation. Complementary DNAs are to be functionally expressed in appropriate mutants of Saccharomyces cerevisiae to test their function.

In a reverse genetics approach, several representative Arabidopsis CDFs will be used in RNA interference technology [10,11]. Regulation and localization of these CDFs need to be investigated by expressing promoter:GUS fusions and epitope-tagged fusion proteins in A. thaliana, and by development and use of specific antibodies. Very little information is available about protein-protein interactions of membrane. Such interactions might be vital for CDF function because their substrate metal cations are thought to be bound to metallochaperone proteins in the cytoplasm.

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