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Željezo je vrlo bitan element za temeljne metaboličke procese. S obzirom na činjenicu da je neophodan za niz funkcija, organizmi ga moraju pohranjivati na način da bude netoksičan i brzo dostupan. To je postignuto „pakiranjem“ željeza u proteinske omotače zvane feritini. Feritini su simetrični proteini koji se sastoje od jezgre izgrađene od kompleksa željezovog oksida i hidroksida te proteinske ljuske. Željezo se iz mononuklearno-fagocitnog sustava (,,reciklaža'' eritrocita) i tankog crijeva (ulazak preko probavnog sustava) do svih stanica prenosi transferinom. On ima izrazito veliki afinitet za slobodno željezo, a u stanicu ulazi endocitozom induciranom receptorom gdje ga ispušta u uvjetima nižeg pH. Mehanizam unutarstanične regulacije željeza precizno je reguliran na razini translacije. Citosolna akonitaza u uvjetima smanjene koncentracije željeza gubi svoj Fe-S klaster i kao takva se može vezati na mRNA feritina i transferinskog receptora. U tom se slučaju sinteza feritina smanjuje (pohrana nije toliko bitna), dok se sinteza transferinskog receptora povećava. Porastom broja receptora na površini stanice u nju ulazi više željeza pa tako stanice koje imaju veće potrebe za tim elementom imati izražen veći broj receptora. Željezo se još može nalaziti u obliku hemosiderina, proteina koji je često pokazatelj viška željeza u organizmu. Ostali bitni proteini koji sadrže željezo su hemoproteini i proteini s klasterima željeza i sumpora. Željezo u hemu najčešće ima funkciju reverzibilnog vezanja kisika i transporta elektrona. U hemoglobinu se nalazi preko 60% ukupnog željeza u organizmu. Hemoproteini, kao i Fe-S proteini, bitnu ulogu ostvaruju i u staničnom disanju kao prijenosnici elektrona. Dakle, željezo je element s mnoštvom različitih funkcija te je stoga neophodan za život svih organizama na Zemlji. Iron is an important element for fundamental metabolic processes. Considering the fact that it is essential for a range of functions, it has to be stored in a non-toxic and accessible form. This is achieved by packing iron into protein shells called ferritins. Ferritins are symmetric proteins consisting of iron oxide-hydroxide core and a protein shell. Iron is tranferred from mononuclear phagocyte system (erythrocyte recycling) and duodenum (absorption by digestive system) to the entire organism by transferrin. Transferrin has extremely high affinity for free iron and is taken up by receptor-mediated endocytosis into endosomes. There it releases iron in the low pH environment. Intracellular iron regulation mechanism is precisely controlled at translation level. When the cell is depleted of iron, cytosolic aconitase loses its Fe-S cluster. Apoenzyme so formed has the ability to bind to mRNAs for transferrin receptor and ferritin, thus reducing ferritin synthesis and increasing transferrin receptor synthesis. With increase of transferrin receptor number on the cell surface, more iron is taken up by the cell. Cells with greater need for iron have more transferrin receptors on their surface. Another example of an iron storage protein is hemosiderin. It is often an indicator of excess iron in the body. Other important iron containing proteins are hemeproteins and iron-sulfur proteins. Heme iron has a function of reversible oxygen binding and electron transport. Hemoglobin contains over 60% of total body iron. Hemeproteins, together with Fe-S proteins, have an important role in cellular respiration as electron carriers. In conclusion, iron is an element with a variety of functions and is therefore essential for all living organisms on Earth. |
