Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Molecules We Are Made Of
Some Small Molecules and Ions
Inorganic Components

Cells contain a vast array of small organic molecules and Metal Ions. Among these are Coenzymes (Chapter 8), which are specific compounds that act in conjunction with Enzymes to catalyze Chemical Reactions that would otherwise barely proceed.

They frequently function as oxidizing agents, accepting electrons from reduced Organic compounds during oxidation and passing them along to oxygen. Coenzymes also transport the electrons required for reduction processes, such as in light-driven Photosynthesis. The Role of adenosine 5'-triphosphate (ATP) and related compounds in intracellular METABOLISM/26.html">Energy Metabolism is exceptionally vital (Supplement 3-A). Hormones and other low-molecular-weight regulatory compounds, as well as a wide range of metabolic intermediates (metabolites), perform crucial Functions within cells.

The solid components of cells consist primarily of the elements C, H, O, N, S, and P. However, when individual Tissues or whole organisms are incinerated, the resulting ash (typically accounting for 3–5% of the total dry weight, though significantly more in mineralized tissues) also contains many other chemical elements. Among cations, the levels of Na⁺, K⁺, Ca²⁺, and Mg²⁺ are relatively high. A 70-kg human body contains 1,050 g of Ca (mostly in bones), 245 g of K, 105 g of Na, and 35 g of Mg. These are followed by iron (3 g), zinc (2.3 g), and rubidium (1.2 g). Of these three elements, iron and zinc are essential, whereas rubidium is likely non-essential.

Other metallic elements are present in The Human Body in amounts of less than 1 g each, yet at least seven of them play a significant role. These include copper (100 mg), manganese (20 mg), and cobalt (~5 mg). The roles of other elements—such as chromium (<6 mg), tin, and vanadium—in higher animals have only recently been identified [101–103]. Nickel, lead, and several other elements are also highly likely to be essential [103].

Among metalloids, the ash is dominated by phosphorus (700 g in humans), sulfur (175 g), and chlorine (105 g). In addition to these three elements, which are vital for all organisms, higher organisms require selenium, fluorine, silicon (Supplement 2-B), and iodine, while plants require boron (Fig. 2-33).

What is the probability that other elements will also prove to be essential? Consider a human erythrocyte—a disk with a volume of ~80 µm³ containing about 3×10⁸ protein molecules (predominantly Hemoglobin). A single erythrocyte contains 7×10⁵ copper atoms and 10⁵ atoms of the essential element tin. It also harbors 2×10⁴ atoms of silver, a toxic metal. Its concentration—exceeding 10^-7 M—is high enough to suggest that silver might serve some important catalytic function. Whether this is true remains unknown; most likely, silver simply enters our bodies because we handle silver objects such as coins, jewelry, and tableware. Erythrocytes also contain boron and aluminum (3×10⁵ atoms each), arsenic (7×10⁵ atoms), lead (7×10⁴ atoms), and nickel (2×10⁴ atoms). Out of all the elements in the periodic table (up to uranium), only four (Ac, Po, Pa, and Ra) are present at an average of less than 1 atom per Cell [101].

Class="center">

FIG. 2-33. Elements essential to living organisms [102] (highlighted in shaded cells). Eleven elements—C, H, O, N, S, P, Na, K, Mg, Ca, and Cl—make up 99.9% of the human body mass. Trace amounts of 13 additional elements are required for the normal physiological function of higher organisms. Higher plants require boron; for animals, microorganisms, and Algae, this element appears to be non-essential.

Some elements traditionally considered non-essential—specifically Cs, Rb, Sr, and Ni (the latter possibly being essential)—are relatively non-toxic. Others, however, are highly toxic, such as Sb, As, Ba, Be, Cd, Pb, Hg, Ag, Tl, and Th.

The ionic composition of tissues and Body Fluids varies widely. The Blood of marine organisms closely resembles seawater in its content of Na⁺, Cl^-, Ca²⁺, and Mg²⁺. The blood of freshwater and terrestrial organisms contains nearly 10 times less Na and Cl^- and several times less Ca and Mg, yet it remains relatively rich in these ions.

In general, cells are rich in K⁺ and Mg²⁺ (especially K⁺) but contain low levels of Na⁺ and Ca²⁺. Among inorganic anions, chloride holds the primary place, though the majority of negative charges are contributed by organic carboxylate and phosphate groups (Table 2-10), many of which are incorporated into Proteins and other macromolecules. According to Ping's estimates, the concentration of amino acid residues belonging to intracellular proteins is typically 1.66 M. Of these residues, 10% bear negatively charged side groups and 8% bear positively charged ones. The resulting net negative charge reaches a concentration of 38 mM [104].