Ribonucleotide reductase
- OMIM: 180410
- UniProt: P23921
- OMIM: 180390
- UniProt: P31350
The ribonucleotide reductase ( RNR often called for short) is an enzyme that forms the last link in the chain of synthesis of DNA building blocks. Reducing the 2' -hydroxy group of nucleotides.
- 7.1 Notes and references
- 7.2 Literature
- 7.3 External links
Importance and occurrence
Ribonucleotide reductase being an essential enzyme in the organism to prepare DNA chips.
Since the enzyme reduces the 2' -hydroxyl group of the nucleotides, it can also be seen as a link between the DNA and RNA world. In this context, it is also speculated that RNR was the enzyme which paved the way from a primitive RNA to today's DNA world. (See also: Chemical Evolution and ribozyme ) How many enzymes catalyzes ribonucleotide reductase and the corresponding reverse reaction ( oxidation of the deoxynucleotide nucleotide ). This reaction, however, plays no role in biological processes.
Biological Function
The ribonucleotide reductase reduces the nucleotides in their respective deoxynucleotides. The enzyme in this case does not differ by which nucleotide is (adenosine, guanosine, cytidine or thymidine phosphates), both di- and the respective triphosphates are reduced. Monophosphates and ( phosphate-free ) nucleosides are not implemented.
There are several classes of ribonucleotide reductase (Class I to Class III). Although all classes catalyze the same reaction, they are structurally very different. The classes are classified according to the manner in which the system generates the radical. Each class still divided into further sub- classes; but this should not be executed for simplicity.
Class I
Class I ( EC 1.17.4.1 ) is the enzyme type, which occurs in humans and is the best studied. The enzymes are aerobic. Class I RNR are composed of two different subunits: R1 and R2.
R1 includes a plurality of binding sites for effectors as well as the binding site ( active site ) in which the nucleotide is reduced. In R2, a radical is stored on a tyrosine. This radical has a surprisingly long half-life of about four days. R1 and R2 have a very low affinity, so that the enzyme is rare as one complex. This is also the reason why it has never managed to crystallize the ribonucleotide reductase as a whole.
For the catalytic step, the radical from the tyrosyl radical in R2 must be transported to a cysteine in R1. The distance is thereby estimated to be 35 angstroms. Presumably, the electron skips several amino acids to cover the ( molecular ratios ) enormously long way.
Class II
Class II RNR ( EC 1.17.4.2 ) are, inter alia in the organism Lactobacillus lehmannii. Here, the in situ free radical is generated by the cobalt-carbon bond is broken in the coenzyme B12. Enzymes of this class are facultative aerobic. You can work both in the presence and in the absence of oxygen.
Class III
Here, the radical is stored on a glycyl side chain. Bacteriophage T4, for example, is working with this class of enzyme. Class III RNR are anaerobic, meaning they only work in an oxygen free environment.
Mechanism of the reduction
The exact sequence of the nucleotide reduction is not yet fully understood. There are clear indications of a radical mechanism, the central step in the formation of a thiyl radical, a sulfur- centered radical, can be made. To store the enzyme a stable free radical, which must be transported at each turn-over in the binding pocket. After the chemical reaction, the radical is recovered and transported back to the memory location '. It is assumed that the electron jumps over a number of amino acids, all of which are connected by hydrogen bonding.
In the following scheme the process is listed with the presumed intermediate steps, as it is currently being discussed in science. Biochemical mechanism of the reduction of a nucleotide First, a radical is transferred to the 3 ' position of the ribose (step from the first to the second image ). Elimination of water at the 2'- position (Fig. 2 → 3) the radical is transferred to two Cysteinketten of the enzyme (Fig. 3 → 4). Then the radical on ribose is transferred back to the original cysteine (Fig. 4 → 6).
Overall, the ribose is reduced to the oxidation of deoxyribose at two cysteine side chains. One has an RNA - DNA building block was prepared. This block is now ready to be incorporated by the DNA polymerase into the DNA double helix.
Regulation
Since the RNC has to be active only in specific phases of cell division, there is, as with most enzymes mechanisms to disable or RNR. In addition, precisely regulated in a complex system to be reduced to the four nucleotides. Here only briefly the activators and effectors to be enumerated, without going too much into the details:
Activators:
- ATP activates the enzyme
- DATP disabled it
Effectors:
- ATP and dATP → CDP / CTP and UDP / UTP be reduced
- DGTP → ADP / ATP
- DTTP → GDP / GTP
Discovery of ribonucleotide reductase
Since the structure elucidation of DNA by James Watson and Francis Crick in 1953, the question of how the cell produces the building blocks for the DNA polymer. Eight years later, an enzyme mixture was first isolated from different cells with high activity to reduce nucleotides to their corresponding deoxynucleotides.
In the 1990s, the three-dimensional structures of the two subunits were determined separately by means of crystal structure investigations. Ask for the mechanism of the docking of the two subunits of R1 and R2 to each other, the radical transport by the enzyme over a relatively large distance away, or after the generation of the radical, in the preparation of the enzyme to be investigated.
The RNR in cancer research and cancer therapy
The ribonucleotide reductase is also the focus of cancer research. Because the enzyme is always required when the cell divides or needs repair DNA damage, the cell is instructed during growth on the RNR. The enzyme is relatively slow with a turnover rate of about 10 s -1. This is not problematic due to the slow rate of division of normal cells, cancer cells are inhibited but that of rapid growth. However, there are cancer cells that increase the conversion rate of RNR by modifications. A drug that blocks exactly these modified enzymes would slow the cancer growth or even stop.
In the treatment of myeloid leukemias (especially for signs of leukostasis ) and other myeloproliferative disorders such as essential thrombocythemia and polycythemia vera ( rubra) is hydroxyurea (eg Syrea ®, Litalir ®), an inhibitor of ribonucleotide reductase, used as a chemotherapeutic agent.