P.A. Cox - Inorganic chemistry (793955), страница 60
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Co, for which the biochemistry is well studied) are present in much lowerconcentrations than many adventitious ones such as Rb or Sr. The active research field of bioinorganic chemistryaims to understand the role of elements, especially metallic ones fulfilling specialized functions.Major elementsMost major elements of life (except P) are readily available in sea water, which may resemble the environment in whichlife began. They fulfill three main functions.
Nonmetallic elements (except Cl) are components of covalently boundmolecules and ions. H, C, N, O and often S are constituents of proteins, and nucleic acids (DNA and RNA) contain P aswell. The chemical forms of these elements differ.
S is normally present in reduced (R-S-H) form (R representing organicJ3—BIONORGANIC CHEMISTRY261Table 1. Essential and toxic elements in biologyaEssentialfor some species, not necessarily humans.form unknown.bChemicalgroups), but the facile oxidation to R-S-S-R gives disulfide bridges, which play a structural role in proteins. P, on theother hand, is always fully oxidized as phosphate. DNA is essentially made of phosphate diesterscomplexed with Mg2+. Adenosine di- and triphosphate, ADP (1: the organic part denoted Ad) and ATP, respectively,are used in energy storage in all types of cell. The forward reactionhas ΔG around +35 kJ mol−1 under physiological conditions of concentration and pH.
Metabolic energy input (e.g. fromoxidation of glucose) is used to drive it. The reverse reaction then provides energy for essential functions such as musclecontraction or the action of the ion pumps mentioned below. An active human may turn over an amount of ATPequivalent to his or her body weight every day.Bicarbonate and phosphate ions are also present in aqueous solution and act as buffers to maintain pH.Metal cations are present in aqueous solution, and are often complexed to anionic biomolecules. For example, Mg2+ is essential for the function of DNA and for the synthesis and utilization of ATP.
Special ‘ion pumps’ maintain muchlower concentrations of Na+ and Ca2+ within cells than in extracellular fluids, and local changes in the concentrations ofthese two cations are used for signaling. The passage of nerve signals is associated with an influx of Na+. Ca2+ formscomplexes with carboxylate groups and acts to alter the conformation of many macromolecules; in particular this ionplays a role in muscle contraction.Another role for some elements is in inorganic solids. Internal skeletons (bones, teeth) are composed mostly ofapatite Ca5(PO4)3(OH) whereas external shells of molluscs are mostly calcium carbonate.
Silica (SiO2) is used as aprotective solid by many single-celled marine plants, and in the brittle hairs of grasses and stinging nettles. Fe3O4 is usedto store iron, and, as it is magnetic, by ‘magnetotactic’ bacteria to sense the direction of the Earth’s magnetic field.262SECTION J—ENVIRONMENTAL, BIOLOGICAL AND INDUSTRIAL ASPECTSTrace metalsEssential elements of the d block are mostly components of specialized metalloproteins, which provide coordinationsites with appropriate ligand atoms (O, N or S) in favorable geometrical arrangements for complexing a particularmetal ion (see Topic E3).
The important element iron is present in heme proteins containing the group shown inFig. 1a and in iron-sulfur proteins containing groups such as the 4Fe–4S cluster shown in Fig. 1c. Manymetalloproteins are enzymes with a catalytic role.Dioxygen chemistryAbsorption of sunlight in green plants by chlorophyll (which contains magnesium; see Topic G3) provides energy forphotosynthesis, which converts CO2 and H2O into organic compounds and dioxygen. Respiration by both plants andanimals provides metabolic energy by oxidation of organic matter using atmospheric O2. Both photosynthesis andrespiration involve complex electron-transfer chains, including redox reactions of organic (e.g. quinones) and inorganiccompounds (e.g. Fe proteins). The terminal step in photosynthesis isand is performed by a unit containing four Mn ions.
The O2-consuming unit in respiration (known as cytochrome coxidase as it obtains electrons from the heme-iron protein cytochrome c) contains both heme-Fe and Cu at the activesite. The many-electron redox step is facilitated by the presence of two or more transition metals with a number ofaccessible redox states, MnII/III/IV, CuI/II and FeII/III/IV with the ferryl (FeIV=O) state which is unusual for Fe (seeTopic H3).Intermediate oxidation states of O, peroxideand superoxideare generally toxic and are undesirable sideproducts of the above reactions.
Superoxide dismutase is a Cu-Zn-containing enzyme catalyzing thedisproportionation oftoand H2O; various catalases and peroxidases act to decompose peroxide.Oxygenase enzymes catalyze specific oxidation reactions by O2. Cytochrome P-450 enzymes perform reactionssuch as R-H→R-OH and involve a ferryl intermediate. Copper-containing oxygenases are generally found outside cells(rather than inside as with Fe; the difference is thought to reflect the later adoption of Cu in evolution) and areespecially important in reactions that form connective tissue such as collagen.Dioxygen transportAround 65% of Fe in the human body is a component of hemoglobin.
The protein molecule contains four heme subunits. The ‘resting state’ has high-spin Fe2+ but coordination of the strong π acceptor ligand O2 changes it to the lowspin form (see Topic H2). This is important for the action of hemoglobin, as the uptake of O2 by one heme sub-unitincreases the O2 affinity of the others, the cooperative effect making the uptake and release more efficient.
High-spin Fe2+ is slightly too large to fit comfortably in the heme ring, but the low-spin ion is smaller and so O coordination causes2the Fe to shift into the ring center. The ‘proximal’ histidine in the Fe coordination sphere (see Fig.
1b) also moves andacts as a means of communicating between sub-units. Another feature of hemoglobin is designed to lessen its affinity forother ligands. The position of the ‘distal’ histidine shown in Fig. 1b forces coordination in a nonlinear geometry. This isfavorable for O2 but not for strongly competing species such as CO and CN−; although these are still very toxic theywould be even more so otherwise.J3—BIONORGANIC CHEMISTRY263Fig. 1. Fe and Co in biomolecules: (a) heme; (b) O2 binding in hemoglobin (see text); (c) [4Fe–4S] center in iron-sulfur proteins; (d) cobalamin (B12)structure.Other metalsIn terms of abundance in the human body, zinc is the most important trace element after iron. As it occurs only as Zn2+ (see Topic G4) it is not redox active, but coordination of molecules to the ‘soft’ cation is involved in many enzymesfor acid catalysis. These include carbonic anhydrase, which convertsto CO2, and enzymes for hydrolysisand condensation reactions of biopolymers.
Zinc-containing proteins also perform regulatory functions; for example,zinc finger proteins recognize specific DNA sequences and are involved in gene function.Nitrogen fixation is performed by specialized microorganisms, which reduce atmospheric N2 to biologicalnitrogen compounds. Normal nitrogenase enzymes contain molybdenum and iron, but less common forms withvanadium are known.Part of the cobalt-containing coenzyme B12 is shown in Fig. 1d. The group X is variable; X=CH3 gives methylcobalamin, which appears to be the only biological compound containing an organometallic metal-carbon bond (seeTopic H10). It performs alkylations and radical-induced rearrangements of biological compounds.264SECTION J—ENVIRONMENTAL, BIOLOGICAL AND INDUSTRIAL ASPECTSToxic and medicinal elementsAlso listed in Table 1 are some notably toxic elements.
Toxicity is a relative term, and many essential elements are toxiceither in excess (e.g. Fe) or if present in the wrong chemical form (e.g. elemental P or Cl). Many of the very toxic elementslisted in Table 1 are heavy metals, that is, post-transition elements of periods 5 and 6 (see Topics G4 and G6). Theseelements have strong complexing ability and an especially strong affinity for sulfur. They may displace essentialelements such as Ca and Fe, and may also disrupt protein structure by breaking S-S bridges.
Once attached to suitableligands they are hard to displace. Chelation therapy is a treatment for heavy metal poisoning using chelating ligandsthat bind very strongly and can remove the elements in complexed form (see Topic E3).Metallic elements used in medicine include lithium for treating manic depressive illness, platinum complexes such ascis-platin (2), which act as antitumor agents by combining with DNA and inhibiting cell division, and gold compoundsused to treat arthritis.Radioactive isotopes are generally harmful to life because of the damaging effects of ionizing radiation. Elementsthat are retained by the body and/or concentrated in specific organs (e.g. Pu in the liver and in bones, I in the thyroidgland) are especially dangerous.
On the other hand, many radioactive isotopes are used in medicine for diagnostic (tracer)and occasionally therapeutic (cancer treatment) purposes. One of the most useful is technetium (Tc), an artificiallymade element with no stable isotopes. The synthesis of Tc complexes designed to ‘target’ particular organs in the bodyis an active research area.Section J—Environmental, biological and industrial aspectsJ4INDUSTRIAL CHEMISTRY: BULK INORGANIC CHEMICALSKey NotesProductionUsesRelated topicsMajor inorganic chemicals include gases (NH3, N2, O2, Cl2, H2), acids (e.g.
HCl,HNO3, H2SO4), alkalis (NaOH, Na2CO3) and phosphates. One important sectoris the chlor-alkali industry, which produces Cl2, NaOH and Na2CO3 from NaCl.The petrochemical and metallurgical industries and agriculture (fertilizers) aremajor users of inorganic chemicals, as are glass and paper making and theproduction of domestic products such as detergents and bleaches.Inorganic reactions and synthesisIndustrial chemistry: catalysts (J5)(B6)ProductionTable 1 shows a selection of the major inorganic chemicals that are produced in annual quantities of many millions oftonnes.
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