Macrominerals

The macromineralsAny of the six essential elements (Na, Mg, K, Ca, Cl, and P) that provide essential ions in body fluids and form the major structural components of the body.—Na, Mg, K, Ca, Cl, and P—are found in large amounts in biological tissues and are present as inorganic compounds, either dissolved or precipitated. All form monatomic ions (Na⁺, Mg²⁺, K⁺, Ca²⁺, Cl⁻) except for phosphorus, which is found as the phosphate ion (PO₄³⁻). Recall that calcium salts are used by many organisms as structural materials, such as in bone [hydroxyapatite, Ca₅(PO₄)₃OH]; calcium salts are also in sea shells and egg shells (CaCO₃), and they serve as a repository for Ca²⁺ in plants (calcium oxalate).

The body fluids of all multicellular organisms contain relatively high concentrations of these ions. Some ions (Na⁺, Ca²⁺, and Cl⁻) are located primarily in extracellular fluids such as blood plasma, whereas K⁺, Mg²⁺, and phosphate are located primarily in intracellular fluids. Substantial amounts of energy are required to selectively transport these ions across cell membranes. The selectivity of these ion pumpsA complex assembly of proteins that selectively transports ions across cell membranes toward the side with the higher concentration. is based on differences in ionic radius (Section 7.2 "Sizes of Atoms and Ions") and ionic charge.

Maintaining optimum levels of macrominerals is important because temporary changes in their concentration within a cell affect biological functions. For example, nerve impulse transmission requires a sudden, reversible increase in the amount of Na⁺ that flows into the nerve cell. Similarly, when hormones bind to a cell, they can cause Ca²⁺ ions to enter that cell. In a complex series of reactions, the Ca²⁺ ions trigger events such as muscle contraction, the release of neurotransmitters, or the secretion of hormones. When people who exercise vigorously for long periods of time overhydrate with water, low blood salt levels can result in a condition known as hyponatremia, which causes nausea, fatigue, weakness, seizures, and even death. For this reason, athletes should hydrate with a sports drink containing salts, not just water.

Group-Transfer Reactions

Trace metal ions also play crucial roles in many biological group-transfer reactionsA reaction in which a recognizable functional group is transferred from one molecule to another.. In these reactions, a recognizable functional group, such as a phosphoryl unit (−PO₃⁻), is transferred from one molecule to another. In this example,

a unit is transferred from an alkoxide (RO⁻) to hydroxide (OH⁻). To neutralize the negative charge on the molecule that is undergoing the reaction, many biological reactions of this type require the presence of metal ions, such as Zn²⁺, Mn²⁺, Ca²⁺, or Mg²⁺ and occasionally Ni²⁺ or Fe³⁺. The effectiveness of the metal ion depends largely on its charge and radius.

Zinc is an important component of enzymes that catalyze the hydrolysis of proteins, the addition of water to CO₂ to produce HCO₃⁻ and H⁺, and most of the reactions involved in DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) synthesis, repair, and replication. Consequently, zinc deficiency has serious adverse effects, including abnormal growth and sexual development and a loss of the sense of taste.

Biological Oxidation–Reduction Reactions

A third important role of trace elements is to transfer electrons in biological oxidation–reduction reactions. Iron and copper, for example, are found in proteins and enzymes that participate in O₂ transport, the reduction of O₂, the oxidation of organic molecules, and the conversion of atmospheric N₂ to NH₃. These metals usually transfer one electron per metal ion by alternating between oxidation states, such as 3+/2+ (Fe) or 2+/1+ (Cu).

Because most transition metals have multiple oxidation states separated by only one electron, they are uniquely suited to transfer multiple electrons one at a time. Examples include molybdenum (+6/+5/+4), which is widely used for two-electron oxidation–reduction reactions, and cobalt (+3/+2/+1), which is found in vitamin B₁₂. In contrast, many of the p-block elements are well suited for transferring two electrons at once. Selenium (+4/+2), for example, is found in the enzyme that catalyzes the oxidation of glutathione (GSH) to its disulfide form (GSSG): 2 GSH + H₂O₂ → 2 H₂O + GSSG. (For more information about glutathione and its disulfide form, see Chapter 4 "Reactions in Aqueous Solution", Section 4.9 "Quantitative Analysis Using Titrations", Example 20.)

Structural Components

Trace elements also act as essential structural components of biological tissues or molecules. In many systems where trace elements do not change oxidation states or otherwise participate directly in biochemical reactions, it is often assumed, though frequently with no direct evidence, that the element stabilizes a particular three-dimensional structure of the biomolecule in which it is found. One example is a sugar-binding protein containing Mn²⁺ and Ca²⁺ that is a part of the biological defense system of certain plants. Other examples include enzymes that require Zn²⁺ at one site for activity to occur at a different site on the molecule. Some nonmetallic elements, such as F⁻, also appear to have structural roles. Fluoride, for example, displaces the hydroxide ion from hydroxyapatite in bone and teeth to form fluoroapatite [Ca₅(PO₄)₃F]. Fluoroapatite is less soluble in acid and provides increased resistance to tooth decay. Another example of a nonmetal that plays a structural role is iodine, which in humans is found in only one molecule, the thyroid hormone thyroxine. When a person’s diet does not contain sufficient iodine, the thyroid glands in their neck become greatly enlarged, leading to a condition called goiter. Because iodine is found primarily in ocean fish and seaweed, many of the original settlers of the American Midwest developed goiter due to the lack of seafood in their diet. Today most table salt contains small amounts of iodine [actually potassium iodide (KI)] to prevent this problem.

An individual with goiter. In the United States, “iodized salt” prevents the occurrence of goiter.