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( Ribonucleotide reductase)
Ribonucleotide reductase (RNR) is an enzyme that controls the cellular concentration of deoxyribonucleotides. Biosynthesis begins with the building up of essential molecules that RNR processes in a catalyzed reaction to make deoxyribonucleotides. RNR assembles deoxyribonucleotides for the synthesis of DNA. The processes are identical in all living organisms. What makes RNR unique from other enzymes is the need for a free radical. The iron-dependent enzyme, ribonucleotide reductase (RNR), is essential for DNA synthesis. Class I RNR comprises RNR1 and RNR2 subunits, which can associate to form an active heterodimeric tetramer. Since the enzyme catalyses the de novo synthesis of deoxyribonucleotides (dNTPs), precursors to DNA synthesis, it is essential for cell proliferation. Each RNR1 monomer consists of three domains one mainly helical domain comprising the 220 N-terminal residues; a second large ten-stranded a/ß structure [a-helix and ß-sheet, Structural Classification of Proteins (SCOP)] comprising 480 residues; and a small five-stranded a/ß structure comprising 70 residues (Jordan & Reichard, 1998). RNR2 contains a diferric iron center and a stable tyrosyl radical. In E. coli, the tyrosyl radical is located at position 122 (Y122) providing the stable radical for the Class I RNR2 subunits (Hogbom et al., 2001). In A. aegypti, this tyrosyl radical is located at position 184 (Y184) (Pham et al., 2002). The tyrosyl radical is deeply buried inside the protein in a hydrophobic environment, located close to the iron center that is used in the stabilization of a tyrosyl radical. The structure of two µ-oxo-linked irons is dominated by ligands that serve as iron binding sites four carboxylates [aspartate (D146), glutamate (E177, E240, and E274)] and two histidines (H180 and H277) (Pham et al., 2002). Association occurs between the C-terminus of RNR2 and the C-terminus of RNR1 (Jordan & Reichard, 1998). Enzymatic activity is dependent on association of the RNR1 and RNR2 subunits. The active site consists of the active dithiol groups from the RNR1 as well as the diferric center and the tyrosyl radical from the RNR2 subunit. Other residues of RNR2, such as aspartate (D273), tryptophan (W48), and tyrosine (Y356) further stabilize the active-site tyrosyl radical thus allowing electron transfer (Jordan & Reichard, 1998). These residues help in the transfer of the radical electron from tyrosine (Y122) of RNR2 to cysteine (C439) of RNR1. The electron transfer begins on RNR2 tyrosine (Y122) and continues in RNR2 to tryptophan (W48), which is separated from RNR1 tyrosine (Y731) by 2.5 nanometers. Electron transfer from RNR2 to RNR1 occurs via tyrosine (Y356 to Y731) and continues on through tyrosine (Y730) to cysteine (C439) in the active site (Chang et al., 2004). Site-directed mutations of the RNR primary structure indicate that all residues cited above participate in the long distance transfer of the free radical to the active site (Jordan & Reichard, 1998).
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Ribonucleotide reductase Subcategories
Ribonucleotide reductase Articles
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