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Organisms have exploited the chemical properties of the elements in remarkable ways,providing examples of coordination specicities that are far higher than observed in simplecompounds. This chapter describes how different elements are taken up selectively bydifferent cells and intracellular compartments and the various ways they are exploited.We discuss the structures and functions of complexes and materials that are formed in thebiological environment in the context of the chemistry covered earlier in the text.

Biological inorganic chemistry (bioinorganic chemistry) is the study of the inorganicelements as they are utilized in biology. The main focus is on metal ions, for which we areinterested in their interaction with biological ligands (structure and dynamics) and theimportant chemical properties they are able to exhibit and impart to an organism. Theseproperties include ligand binding, catalysis, signalling, regulation, sensing, defence, andstructural support.

The organization of cells

To appreciate the role of the elements (other than C, H, O, and N) in the structure andfunctioning of organisms we need to know a little about the organization of the atomof biology, the cell, and its fundamental particles, a cells constituent organelles.

26.1 The physical structure of cellsKey points: Living cells and organelles are enclosed by membranes; theconcentrations of specific elements may vary greatly between different compartmentsdue to the actions of ion pumps and gated channels.

Cells, the basic unit of any living organism, range in complexity from the simplest typesfound in prokaryotes (bacteria and bacteria-like organisms now classied as archaea) andthe much larger and more complex examples found in eukaryotes (which include animalsand plants). The main features of these cells are illustrated in the generic model shownin Fig. 26.1. Crucial to all cells are membranes, which act as barriers to water and ionsand make possible the management of all mobile species and of electrical currents.Membranes are lipid bilayers, approximately 4 nm thick, into which are embeddedprotein molecules and other components. Bilayer membranes have great lateral strengthbut they are easy to bend. The long hydrocarbon chains of lipids make the membraneinterior very hydrophobic and impermeable to ions, which must instead travel throughspecic channels, pumps, and other receptors provided by special membrane proteins.The structure of a cell also depends on osmotic pressure, which is maintained by highconcentrations of solutes, including ions, imported during active transport by pumps.

Prokaryotic cells consist of an enclosed aqueous phase, the cytoplasm , which containsthe DNA and most of the materials used and transformed in the biochemical reactions.Bacteria are classied according to whether they are enclosed by a single membraneor have an additional intermediate aqueous space, the periplasm , between the outer

Biological inorganic

chemistry 26The organization of cells

26.1 The physical structure of cells

26.2 The inorganic composition of cells

Transport, transfer, andtranscription

26.3 Sodium and potassium transport

26.4 Calcium signalling proteins26.5 Zinc in transcription

26.6 Selective transport and storage of iron

26.7 Oxygen transport and storage

26.8 Electron transfer

Catalytic proceses

26.9 Acidbase catalysis

26.10 Enzymes dealing with H 2 O2 and O 2

26.11 The reactions of cobalt-containing enzymes

26.12 Molybdenum and tungsten enzymes

Biological cycles

26.13 The nitrogen cycle

26.14 The hydrogen cycle

Sensors

26.15 Iron proteins as sensors

26.16 Proteins that sense Cu and Zn levels

Biomineralization

The chemistry of elements inmedicine

26.17 Chelation therapy

26.18 Cancer treatment

26.19 Anti-arthritis drugs

26.20 Imaging agents

Perspectives

26.21 The contributions of individualelements

26.22 Future directions

FURTHER READINGEXERCISESPROBLEMS

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membrane and the cytoplasmic membrane, and are known as Gram-positive or Gram-negative , respectively, depending on their response to a staining test with the dye crystalviolet. The much more extensive cytoplasm of eukaryotic cells contains subcompart-ments (also enclosed within lipid bilayers) known as organelles , which have highly specialized functions. Organelles include the nucleus (which houses DNA), mitochon-dria (the fuel cells that carry out oxidative respiration), chloroplasts (the photocellsthat harness light energy), the endoplasmic reticulum (for protein synthesis), Golgi(vesicles containing proteins for export), lysosomes (which contain degradative enzymesand help rid the cell of waste), peroxisomes (which remove harmful hydrogen peroxide),and other specialized processing zones.

26.2 The inorganic composition of cellsKey points: The major biological elements are oxygen, hydrogen, carbon, nitrogen,phosphorus, sulfur, sodium, magnesium, calcium, and potassium. The trace elementsinclude many d metals, as well as selenium, iodine, silicon, and boron.

Table 26.1 lists many of the elements known to be used in living systems, although notnecessarily by higher life forms. All the 2 p and 3 p elements except Be, Al, and the noblegases are used, as are most of the 3d elements, whereas Br, I, Mo, and W are the only heavier elements so far conrmed to have a biological function. Several others, such as Li,Tc, Pt, and Au, have important and increasingly well-understood applications inmedicine.

The biologically essential elements can be divided broadly into two classes, major andtrace . Although a good idea of the biological abundances of different elements is given inTable 26.1, the levels vary considerably among organisms and different components of organisms. For example, Ca has little role in microorganisms but is abundant in higherlife forms, whereas the use of Co by higher organisms depends upon it being incorporatedinto a special cofactor (cobalamin) by microorganisms. There is probably a universalrequirement for K, Mg, Fe, and Mo. Vanadium is used by lower animals and plants aswell as some bacteria. Nickel is essential for most microorganisms, and is used by plants,

Table 26.1 The approximate concentrations (mol dm 3 ), where known, of elements(apart from C, H, O, N, P, S, Se, Br, I, B, Si and W) in different biological zones

Element External fluids(sea water)

Free ions inexternal fluids(blood plasma)

Cytoplasm(free ions)

Comments on statusin cell

Na > 10 1 10 1 < 10 2 Not boundK 10 2 4 10 3 3 10 1 Not boundMg > 10 2 10 3 %10 3 Weakly bound as

ATP complexCa > 10 3 10 3 %10 7 Concentrated in

some vesiclesCl 10 1 10 1 10 2 Not boundFe 10 17 (Fe(III)) 10 16 (Fe(III)) < 10 7 (Fe (II)) Too much unbound Fe is

toxic (Fenton chemistry)in and out of cells

Zn < 10 8 10 9 < 10 11 Totally bound, butmay be exchangeable

Cu