Plant Physiology - Musienko M.M. 2001

Root Nutrition of Plants
Metal-Organic Coordination Compounds

Mineral elements (so-called non-organogens) account for approximately 5% of the Elemental Composition of plant organisms. Among them, ten metals (sodium, potassium, magnesium, calcium, manganese, iron, cobalt, copper, zinc, and molybdenum) are known as the "metals of life" or "biometals." However, the list of biometals should be broader to include vanadium, nickel, and cadmium, which have been detected in Proteins. The evolutionary principles behind the Selection of these biometals for the construction of biological systems remain poorly understood, although it is quite clear that their natural Abundance is not the primary criterion. For instance, silicon and aluminum are quite abundant in nature, yet they are not considered elements of life, whereas molybdenum is relatively scarce, yet essential for the vital activity of organisms.

In D. I. Mendeleev's periodic table, biometals are located relatively compactly, forming two distinct clusters (Na, K, Mg, Ca and Mn, Co, Fe, Cu, Zn). The differences in the PHYSICOCHEMICAL PROPERTIES OF these metals largely determine the specific nature of their effects on living organisms. For example, the sodium ion is predominantly extracellular, whereas potassium is an intracellular element. The tendency to form covalent bonds is very weakly expressed in these elements; consequently, they do not participate in redox processes and cannot be replaced by similarly sized ions, such as copper ions, which readily form covalent bonds and take part in redox reactions.

The tendency to form covalent bonds is more pronounced in magnesium than in calcium, allowing magnesium to form fairly stable donor-acceptor bonds with nitrogen atoms in variable groups.

Numerous plant macromolecules interact with Metal Ions. Natural ligands include proteins, Polypeptides, Nucleic Acids, and Vitamins. Each of these complexes consists of a central ion (most commonly a metal), metal-binding groups, and counterions, which are typically bound within the complex via electrostatic forces.

The metal and the Ligand together form the inner sphere of the complex. Functional groups capable of readily coordinating with transition metal ions include carboxyl, amino, imino, indole, hydroxyl, sulfhydryl, and phosphate groups. It should be noted that the stability of metal-ligand complexes depends on the presence of cyclic structures. Even The formation of a closed ring with the same donor atoms increases the Stability of the complex by several orders of magnitude.

The most widespread natural ligands include phytin, chlorophyll, plastocyanin, and iron-containing compounds (ferredoxin, Cytochromes). The primary Functions of metals within complexes are participation in redox reactions and the Introduction/15.html">Regulation of enzyme Activity. Based on evolutionary biochemistry data, metals initially fulfilled these functions via free ions, later through compounds with low-molecular-weight proteins and NUCLEOTIDES, subsequently through high-molecular-weight proteins and nucleotides, and finally via Lipoproteins.

Experimental studies on the involvement of metals in The regulation of enzyme activity have provided valuable insights not only into the mechanisms of enzymatic reactions but also into the specific functions of mineral elements in plants. Drawing upon the fundamental principles of Enzymatic Catalysis, Allosteric Regulation is widely recognized as the primary mechanism by which metals modulate catalytic activity. Allosteric phenomena involve structural alterations associated with Regions of the polypeptide chain distant from the Active Site. Therefore, metals can be regarded as allosteric effectors of catalysis. For instance, magnesium acts as an allosteric activator of alkaline hexosediphosphatase.

Metal ions directly influence the conformational METABOLISM/8.html">Properties of Enzymes and serve as convenient investigative probes within the active site. Furthermore, metal ions maintain the Structural Organization of the enzyme and alter the conformation of both the enzyme and the substrate. Even minor substrate Conformational changes induced by metals can lead to significant alterations in enzymatic activity. The direct participation of ions in the catalytic act has been established for numerous redox enzymes. Enzymes containing several interacting transition metal atoms (iron, copper, molybdenum, or manganese) within a single macromolecule readily donate and accept electrons. For example, molybdenum within plant nitrate reductase serves as a component of the Electron Transport Chain from reduced pyridine nucleotide to nitrate. It should be noted that when directly participating in the catalytic act, the metal contacts both the protein and the substrate. In such cases, the function of the metal is multifaceted; for instance, it facilitates substrate attachment to the enzyme (such as monovalent cations acting as bridges in phosphorylation reactions).

In the case of flavin enzymes, metals are believed to assist both in attaching the substrate to the protein moiety and in mediating the relationship between the flavin group and the apoenzyme. Examples of such enzymes include nitrate reductase (iron, molybdenum), nitrite reductase (iron, copper), and hydroxylamine reductase (iron, manganese). Most frequently, the metal-substrate complex acts as the true substrate in enzymatic reactions (e.g., nucleotides). The primary function of the cation in such cases is to neutralize the negative charge on the phosphate residue of the substrate (it is generally accepted that Mg⁺⁺ — ATP is the true substrate for this enzyme).

Even if ions are not part of the active site Structure, they can influence the dissociation constant of the enzyme's functional groups, alter the local charge distribution on the molecular surface, and affect the overall net charge of the entire molecule. A number of divalent cations are believed to participate in stabilizing the active conformation of plant enzymes. Such a function is performed, for example, by copper in spinach ribulose bisphosphate carboxylase and by calcium in horseradish peroxidase.

Certain metals essential for enzymatic activity (copper, iron, manganese, nickel) frequently exhibit catalytic activity even in the form of Inorganic Compounds. For instance, copper ions catalyze The oxidation of ascorbic acid, iron ions accelerate the decomposition of hydrogen peroxide, and various metal ions are capable of decarboxylating certain dicarboxylic acids. Nevertheless, the catalytic efficiency of these ions increases by several orders of magnitude when the metal is bound to the protein moiety of the enzyme molecule.

One of the reasons for altered enzyme activity under The Influence of Mineral Nutrition elements is a shift in THE SPECTRUM OF molecular forms of these elements. For example, nitrogen alters the spectrum of Glutamate dehydrogenase, whereas magnesium deficiency in maize induces the appearance of a new Lactate dehydrogenase isoenzyme, alters the number of malate dehydrogenase and glucose-6-phosphate dehydrogenase Isoenzymes, and increases the quantity of Alcohol dehydrogenase isoenzymes.

Alterations in the mineral COMPOSITION OF THE nutrient medium lead to Changes in the quantitative ratio of individual ions within the plant itself, which subsequently affects enzymatic activity. It has been established that zinc deficiency in the soil retards the uptake of Macronutrients while simultaneously stimulating The activity of Glutamate Dehydrogenase and Carbonic anhydrase. A similar phenomenon has been observed when iron levels in the nutrient medium are reduced.

Consequently, based on the aforementioned facts, several potential Mechanisms for the involvement of metals in enzymatic catalysis can be postulated:

·by binding at the Active Site of the enzyme, metal ions participate in the Formation of the enzyme-substrate complex (in this case, they constitute a structural part of the enzymes);

·metals act as Inducers and stabilizers of the active enzyme conformation (in such cases, they may bind to protein regions located at a distance from the active site);

·metals link two or more monomeric protein subunits into a catalytically active polymeric aggregate.

The critical significance of certain cations for specific stages of Protein Biosynthesis is beyond doubt, although The Role of inorganic ions in The biosynthesis of plant enzymes remains insufficiently investigated. The participation of both divalent ions (magnesium, manganese) and monovalent ions, including ammonium, in regulating the RNA polymerase system has been convincingly demonstrated. The role of ions in enzyme biosynthesis lies in their influence on aminoacyl-tRNA synthetase activity and the maintenance of ribosome structure. It is suggested that divalent metals play a role in the specific selection of the enzyme's binding site on DNA.

An interesting example of metal ion participation in non-enzymatic catalysis is the catalysis of phosphate transfer from ATP to other acceptors. The primary function of metal ions in non-enzymatic polyphosphate transformations is to neutralize the negative charges on the polyphosphate, thereby facilitating nucleophilic attack (magnesium ions serve as a classic example). The reaction rate increases in proportion to the concentration of the metal ion in the medium.

Therefore, mineral nutrition elements are capable of associating not only with the Major Classes of Organic compounds but also with energetically crucial compounds such as ATP, thereby influencing Energy Metabolism in plants. Without resolving the questions regarding The impact of these elements on organic compounds, establishing a comprehensive theoretical framework for mineral nutrition remains impossible.