Metalloproteins are biomolecules containing metal ions. From transporting oxygen and cell signals to catalyzing chemical reactions, metalloproteins are essential in the biochemistry of nature and health.
Found in all life forms, including plants, animals, bacteria and archaea, metalloproteins are proteins with one or more metal ions. The ion, bound in the protein's three-dimensional architecture, is an influential cofactor.
It participates in chemical reactions or affects the protein's shape and activity. Metals used are countless, with iron, copper, zinc, magnesium, manganese, molybdenum, cobalt and nickel among the most common.
Each metal has unique chemical properties aligned to the diverse functions of metalloproteins. Metalloproteins can be classified into categories such as hemoproteins, with heme groups, cuproproteins which contain copper.
Heme, or haem, is a ring-shaped iron-containing molecular component of hemoglobin, necessary to bind oxygen in the bloodstream. Copper helps make hemoglobin, fortifies immune system and maintains nerve cell health.
Metals have many advantages in biochemistry. Their ability to readily accept or donate electrons enables oxidation-reduction (redox) reactions, fundamental for energy production and other metabolic processes.
As Lewis acids or pH acids, metals catalyze reactions by coordinating with and activating substrate molecules. The metal ion's charge and coordination geometry are refined by the surrounding protein.
Health and Disease
Malfunction or imbalances in metal homeostasis cause a range of health problems.
Iron: Iron deficiency anemia, caused by insufficient ferritin, an iron storage protein, is a common health problem. Iron overload, or hemochromatosis, can damage organs due to accumulation of iron-rich proteins.
Copper: Wilson's disease, a genetic disorder affecting copper metabolism, causes toxic accumulation of copper in the liver, brain, and other organs. Menkes disease arises from poor copper absorption.
Zinc: Zinc deficiency can impair immune function, wound healing, and cognitive development.
Molybdenum: Molybdenum cofactor deficiency is a rare genetic disorder. It disrupts activity of several molybdenum-dependent enzymes, causing neurological dysfunction.
Functions of Metalloproteins
The functions of metalloprotein are countless. A few examples are:
Oxygen Transport: Hemoglobin, an iron-containing protein found in red blood cells, transports oxygen from the lungs to the tissues. Myoglobin, a similar protein in muscle tissue, stores oxygen for energy production.
Electron Transfer: Cytochromes, containing heme groups with iron, play essential roles in the electron transport chain in mitochondria of cells. They facilitate the transfer of electrons, generating energy in the form of ATP.
Catalysis: Enzymes known as metalloenzymes rely on metal ions to catalyze reactions. Carbonic anhydrase, containing zinc, accelerates conversion of carbon dioxide to bicarbonate, necessary for respiration and pH regulation.
Nitrogenase, containing iron and molybdenum, is responsible for nitrogen fixation, converting atmospheric nitrogen into ammonia. Plants can't take up nitrogen as is, thus need this conversion to get vital nutrients.
Antioxidant Defense: Superoxide dismutase (SOD), containing copper, zinc, manganese, or iron, protects cells from damaging free radicals by converting superoxide into less harmful substances.
Signal Transduction: Some metalloproteins are signaling molecules, or regulate activity of other proteins involved in cell communication.
Metalloproteins in Photosynthesis
Photosynthesis is an example of metalloproteins in action. Chlorophyll-bound metalloproteins convert light energy into chemical energy. The magnesium in chlorophyll absorbs light and helps synthesize glucose.
Plants produce oxygen and organic compounds necessary for most life on Earth. Sixty percent of atmospheric oxygen comes from phytoplankton, ancient organisms with similar mechanisms.
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