About Metalloproteins

Metalloproteins have captivated chemists and biochemists, particularly since the 1950s, when the first X-ray crystal structure of a protein, sperm whale myoglobin, indicated the presence of an iron atom. They account for nearly half of all proteins in nature. Protein metal-binding sites are responsible for catalyzing important biological processes, such as photosynthesis, respiration, water oxidation, molecular oxygen reduction and nitrogen fixation. This is indicated by the active-site structures of the enzymes involved, which are often reminiscent of minerals. Although the reactions are based on metal centres, the protein matrix regulates reactivity .Much effort has been devoted to understanding the structure and function of these proteins. The ultimate test is to identify new metalloproteins if sequence is known.
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. Metalloproteins have many different functions in cells, such as enzymes, transport and storage proteins, and signal transduction proteins. The metal ion is usually coordinated by nitrogen, oxygen or sulfur atoms belonging to amino acids in the polypeptide chain and/or a macro cyclic ligand incorporated into the protein. The presence of the metal ion allows metalloenzymes to perform functions such as redox reactions that cannot easily be performed by the limited set of functional groups found in amino acids.

Evolution of Metalloproteins

The most popular autotrophic theory of the origin of life postulates that primordial metabolisms developed on mineral iron–sulphur surfaces under reducing conditions. During this period of the Earth's evolution, between 4.6 and 3.5 billion years  ago, the atmosphere was probably rich in gases such as H2, CO and CO2, and its hot oceans contained relatively high concentrations of transition-metal ions such as Fe2+ and Ni2+.
The physical and chemical properties of a selected metal satisfied a protein's need to form structure, as for zinc-fingers, or to drive catalysis. Proteins evolved to use those metals that at least once were, most accessible. A tenet of the cell biology of metals is that some metals tend to bind organic molecules more avidly than others. The natural order of stability for divalent metals, often called the Irving–Williams series, sets out a resulting trend with copper and zinc forming the tightest complexes, then nickel and cobalt, followed by ferrous iron and manganese and finally, forming the weakest complexes, calcium and magnesium.   This analogy constitutes the basis for the concept of primordial surface metabolism on metalloproteins theory for the origin of life.

Metalloprotein in Life

Metal ions are common cofactors. The study of these cofactors falls under the area of bioinorganic chemistry. In nutrition, the list of essential trace elements reflects their role as cofactors. In humans this list commonly includes iron, manganese, cobalt, copper, zinc, and molybdenum.  Although chromium deficiency causes impaired glucose tolerance, no human enzyme that uses this metal as a cofactor has been identified . Iodine is also an essential trace element, but this element is used as part of the structure of thyroid hormones rather than as an enzyme cofactor. Calcium is another special case, in that it is required as a component of the human diet, and it is needed for the full activity of many enzymes such as nitric oxide synthase, protein phosphatases or adenylate kinase, but calcium activates these enzymes in allosteric regulation, often binding to these enzymes in a complex with calmodulin. Calcium is therefore a cell signaling molecule, and not usually considered as a cofactor of the enzymes it regulates.
Other organisms require additional metals as enzyme cofactors, such as vanadium in the nitrogenase of the nitrogen-fixing bacteria of the genus Azotobacter, tungsten in the aldehyde ferredoxin oxidoreductase of the thermophilic archaean Pyrococcus furiosus,and even cadmium in the carbonic anhydrase from the marine diatom Thalassiosira weissflogii. .In many cases, the cofactor includes both an inorganic and organic component. One diverse set of examples are the haem proteins, which consists of a porphyrin ring coordinated to iron.
Thus metalloproteins are critical for protein structure, function and stability. For this reason, many chemists and biologists are determined to understand the mechanisms and the cellular roles of these remarkable enzymes.