zinc
Definition: Zinc is a metallic chemical element with the symbol Zn and atomic number 30. It is a first-row transition metal in group 12 of the periodic table.
Zinc is chemically similar to magnesium because its ion is of similar size and its only common oxidation state is +2.
Zinc is the 24th most abundant element in the Earth’s crust and has five stable isotopes.
Biological role
Zinc is an essential trace element, necessary for plants, animals, and microorganisms. Zinc is found in nearly 100 specific enzymes (other sources say 300), serves as structural ions in transcription factors and is stored and transfered in metallothioneins.
It is "typically the second most abundant transition metal in organisms" after iron and it is the only metal which appears in all enzyme classes.
The human body has 2-4 grams of zinc distributed throughout the body.
Most zinc is in the brain, muscle, bones, kidney, and liver, with the highest concentrations in the prostate and parts of the eye. Semen is particularly rich in zinc, which is a key factor in prostate gland function and reproductive organ growth.
In humans, zinc plays "ubiquitous biological roles". It interacts with "a wide range of organic ligands" and has a role in the metabolism of RNA and DNA, signal transduction, and gene expression. It also regulates apoptosis and can "modulate brain excitability".
A 2006 study estimated that about 10% of human proteins (2800) potentially bind zinc in addition to hundreds which transport and traffic zinc. A similar in silico study in the plant Arabidopsis thaliana found 2367 zinc-related proteins.
Enzymes
Zinc is a good Lewis acid, making it a useful catalytic agent in hydroxylation and other enzymatic reactions. The metal also has a flexible coordination geometry, which allows proteins using it to rapidly shift conformations to perform biological reactions.
Two examples of zinc-containing enzymes are carbonic anhydrase and carboxypeptidase, which are vital to the processes of carbon dioxide (CO2) regulation and digestion of proteins, respectively.
In vertebrate blood, carbonic anhydrase converts CO2 into bicarbonate and the same enzyme transforms the bicarbonate back into CO2 for exhalation through the lungs.
Without this enzyme, this conversion would occur about one million times slower at the normal blood pH of 7 or would require a pH of 10 or more. The non-related β-carbonic anhydrase is required in plants for leaf formation, the synthesis of indole acetic acid (auxin) and anaerobic respiration (alcoholic fermentation).
Carboxypeptidase cleaves peptide linkages during digestion of proteins. A coordinate covalent bond is formed between the terminal peptide and a C=O group attached to zinc, which gives the carbon a positive charge. This helps to create a hydrophobic pocket on the enzyme near the zinc, which attracts the non-polar part of the protein being digested.
Zinc fingers
Zinc serves a purely structural role in zinc fingers, twists and clusters.
Zinc fingers form parts of some transcription factors, which are proteins that recognize DNA base sequences during the replication and transcription of DNA (zinc finger proteins, ZNFs).
Each of the nine or ten Zn2+ ions in a zinc finger helps maintain the finger’s structure by coordinately binding to four amino acids in the transcription factor.
The transcription factor wraps around the DNA helix and uses its fingers to accurately bind to the DNA sequence.
Metabolism
In blood plasma, zinc is bound to and transported by albumin (60 %, low-affinity) and transferrin (10%).
Since transferrin also transports iron, excessive iron reduces zinc absorption, and vice-versa. A similar reaction occurs with copper.
The concentration of zinc in blood plasma stays relatively constant regardless of zinc intake.
Cells in the salivary gland, prostate, immune system and intestine use zinc signaling as one way to communicate with other cells.
Zinc may be held in metallothionein reserves within microorganisms or in the intestines or liver of animals.
Metallothionein in intestinal cells is capable of adjusting absorption of zinc by 15-40%.
However, inadequate or excessive zinc intake can be harmful; excess zinc particularly impairs copper absorption because metallothionein absorbs both metals.
Pathology
zinc deficiency
Links
References
Prasad AS. Zinc in human health: effect of zinc on immune cells. Mol Med. 2008 May-Jun;14(5-6):353-7. PMID:18385818#