Nucleotide binding proteins, iron protein

Inspection of the size of the various nucleotide-binding proteins, whether coenzyme B12- or iron subunit-associated, reveals a preference for the 75,000 to 100,000 dalton range. The 45,000 dalton L1 subunits of rat liver reductase with one half this value still fit the picture although in no other case dissociation into two polypeptide chains has been found. One, two, or four of these large subunits can apparently form a holoenzyme like in the Lactobacillus (1), Corynebacterium, E. coli, Scenedesmus, mammalian enzymes... 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

Angerer, A., Gaisser, S. and Braun, V. (1990). Nucleotide sequences of the sfuA, sfuB, and sfuC genes of Serratia marcescens suggest a periplasmic-binding-protein-dependent iron transport mechanism, J. Bacteriol., 172, 572-578. 

Figure 4.5 (a) the positions of the three amide NH atoms in the anion-binding nest motif found in a wide range of functional anion binding proteins, (b) the five-amide compound nest in the P-loop of p21 ras - a nucleotide triphosphate binding protein and (c) a six-amide compound nest surrounding the iron sulfur core in ferredoxin. (Reproduced from Section Key Reference with permission from Elsevier). 

Figure 12-4 Differential binding of IRPl and IRP2 to natural IREs (Iron Responsive Elements). P-5 -RNAs (n = 29-30 nucleotides) were melted and annealed before mixing with recombinant IRP proteins (The proteins were kindly provided by E. A. Leibold, University of Utah, and W. E. Walden, University of Illinois). RNA-protein complexes were separated from RNA by electrophoresis in non-denatuiing polyacrylamide gels [20]. Per contains an internal loop/bulge (Figure 12-3), and TfR, eALAS, and m-aconitase IREs have C-bulges. Per mutation AU6 converts the Per internal loop/bulge to a C-bulge. Per ferritin TfR transferrin receptor eALAS erythroid amino-levulinate synthase and m-aconitase. No IRE/IRP complex was detectable.

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