Zinc-dependent enzyme functions
Zinc is one of the most important trace elements due to its ubiquitous participation in the most diverse biological reactions. The essential vital element is a component or cofactor of more than 200 enzymes and proteins known to date. Zinc is relevant for the configuration of non-enzymatic proteins and fulfills structural, regulatory and catalytic roles in a variety of metalloenzymes such as:
- DNA and RNA polymerases
- Carboanhydrases – rapid release and exhalation of carbon dioxide, a deficiency of carbonic anhydrase due to a zinc deficit promotes an acid metabolic state
- Dehydrogenases
- Oxidoreductases
- Transferases
- Hydrolases
- Isomerases
- Ligases
- Alkaline phosphatases
- Phospholipases
For example, zinc is involved with alcohol dehydrogenase in the breakdown of alcohol in the liver and with alkaline phosphatase in the formation of bone substance. The pancreatic carboxypeptidase and alpha-amylase require the essential trace element for protein digestion. It is true for all zinc-dependent enzymes that they react to a zinc deficiency with a loss of activity. A deficit of zinc seems to cause an insufficient induction of these enzymes. This explains the importance of zinc for, among other things, protein, fat and carbohydrate metabolism, for the acid-base balance, as well as the multitude of functional disorders in zinc deficiency. Other important enzyme functions:
- Protection against free radicals – through superoxide dismutase and zinc thionein.
- Body defense – in cellular and humoral immunity.
- Nucleic acid metabolism – stabilizes the structures of RNA, DNA and ribosomes, protects them from oxidation and promotes differentiation and gene expression.
- Cell proliferation in injury, as well as developmental, growth and regeneration processes – during thymidine incorporation in cell cultures, fetal development, liver regeneration, wound healing and burns, high amounts of zinc are required
- Blood formation
- Maintenance of membrane and protein structures
- Control of protein biosynthesis (new protein formation) via zinc-dependent transcription factors – Zinc is particularly essential for the metabolism of the amino acid cysteine, which is found in the skin and hair.
- Fatty acid and prostaglandin metabolism.
- Neurotransmitter metabolism in the brain
- Sensory functions – seeing, hearing, smelling and tasting.
- Structure and degradation of connective tissue.
- Metabolism of sex hormones – gonads and reproduction.
- Reduction of intestinal absorption of toxic heavy metals, such as lead, cadmium and mercury.
Functions in neurotransmitter metabolism
Zinc is involved in the formation and degradation of various neurotransmitters, especially glutamate and gamma-aminobutyric acid (GABA). In addition, the trace element modulates amino acid receptors, especially NMDA receptors (glutamate receptors), which reduces increased excitability caused by glutamate. This function plays an essential role in epilepsy and febrile convulsions, for example. Furthermore, zinc influences the activity of glutamate decarboxylase, which is important for the synthesis of gamma-aminobutyric acid. GABA represents the most important inhibitory neurotransmitter. In zinc deficiency, the formation of GABA is restricted, which eventually leads to increased excitability of neurons.
Hormonal functions
Zinc is an indispensable vital substance for the synthesis, storage, and secretion of insulin in the beta cells of the endocrine pancreas and is required for its action on the cell. A good supply status of the body with zinc is also essential for the function of proteohormones (hormones with protein character) such as glucagon, gonadotropin, growth hormone and sex hormones. In addition, the trace element is involved in the metabolism of thyroid hormones, growth hormones and prostaglandins. Zinc continues to be essential for the formation of testosterone, thereby influencing the development and maturation of male sex organs as well as spermatogenesis. Likewise, zinc plays an important role in female fertility.Zinc forms dissociable zinc-protein-hormone complexes with pituitary gonadotropins, luteinizing hormones (LH) and steroid hormones (SH), among others, which are important for women. This stimulates the activity of these hormones.
Antioxidant functions
Zinc possesses acute and chronic antioxidant properties based on the sustained induction of antioxidant substances. These include the induction and maintenance of cellular concentrations of the hydroxyl radical scavenger metallothionein and reduced glutathione. In this way, cells are protected from radical attack and their membranes are stabilized. Since zinc is an antagonist of iron and copper, it reduces their reactivity in the process of radical formation. Furthermore, zinc inhibits the absorption of lead and cadmium, protecting against intoxication with these pollutants from the environment. Finally, in the case of zinc deficiency, the erythrocytes (red blood cells), among others, are insufficiently protected against radical attacks. Likewise, increased lipid peroxidations are observed as a result of a zinc deficiency. Substitution with zinc can largely normalize such changes. In addition, zinc is extremely important for other cell membrane-associated functions (cell communication) and structures (cytoskeleton). The cytoskeleton is a scaffold of elastic structures-microtubules and microfilaments-that provides external support to the cell and serves as a substrate for transport and metabolic processes.
Immunomodulatory effects
T helper, T killer, and natural killer cells are essential for optimal immune system function. Normal activity of these T cells requires an adequate supply of zinc. Likewise, the zinc status in the body influences the formation of several lymphokines, which affect the growth, differentiation and activity of the cells of the immune system. Macrophages, which strengthen the immune defense, develop their full effect only with sufficient zinc supply. They are able to remove foreign germs and substances from the body and produce antibodies (monocyte-macrophage system). Experimental findings have shown that zinc salts inhibit the replication of rhinoviruses and protect cell components from damage by bacterial toxins. Zinc deficiency in animals leads to thymic atrophy, which prevents the maturation of T lymphocytes (certain group of white blood cells), into specialized cells of the immune system. Eventually, the number of T lymphocytes decreases. In addition, the production of thymic hormones, which are significant for the differentiation and development of T lymphocytes, is reduced. Stress caused by zinc deficiency and a resulting increase in adrenal steroids are also partly responsible for the decrease in T cells.
Functions in vitamin A metabolism
Zinc is an essential vital substance for the visual process. As a component of alcohol dehydrogenase, zinc is involved in the conversion of retinol to retinal. In addition, the trace element is required for the synthesis of retinol-binding protein (RBP), which transports vitamin A and retinol, respectively, from the liver to other organs or tissues, particularly the retina.
Functions for skin and skin appendages
Skin lesions in zinc deficiency have confirmed that zinc is essential for the normal function of skin and its appendages, such as hair and nails. In particular, the trace element is of considerable importance for the transformation processes of the skin from the stratum germinativum (innermost layer) to the stratum corneum (outer layer, actual skin surface). Zinc also influences the dehydration of linoleic to linolenic acid. These essential fatty acids are responsible for a regulated cornification of the skin. Furthermore, zinc is relevant for the cystine metabolism of the hair root and thus significantly involved in the structural strength of hair as well as nails.
