DNA polymerase III holoenzyme

DNA polymerase III is an enzyme which catalyzes the synthesis of DNA from a DNA template to deoxyribonucleotides. Is a protein complex. Holoenzyme plays the most important role in the prokaryotic DNA replication. Most important features are its many subunits and the very high catalytic activity, accuracy and process activity ( ability of an enzyme to catalyze many reactions after another without losing the substrate ). Addition of DNA polymerase III with two other DNA polymerases are known in prokaryotes.

Function

DNA polymerase III linked several thousand Phosphordiestherbindungen with its substrate before leaving this. Thus, it can hold the die and not released until after the full replication. Furthermore, the DNA polymerase III has a strong catalytic effect which enables it to add 1,000 nucleotides per second. This strong catalyst performance can be explained by the fact that they do not have to detach from the substrate such as DNA polymerase I. The new strand grows in 5 '→ 3' direction. Moreover, it is to read the DNA polymerase III possible 3 '→ 5' correction and wrong to replace incorporated nucleotides.

Construction

The holoenzyme is compared with the DNA polymerase I, an order of magnitude more severe and the molecular weight is close to 900 kDa. The enzyme is constructed as an asymmetrical dimer to replicate both parental strands in the same place at the same time. The asymmetry arises from the fact that the leading and lagging strand are synthesized differently.

The α - subunit is the polymerase and ε is the 3 ' → 5' exonuclease for proofreading. Both are catalytically active but not processive, which assume the subunits β and τ. The processivity can be explained by the spatial structure of β. This subunit forms a ring through which the DNA double-strand slide through and so does not have to separate from the substrate.

Synthesis

The ATP -driven helicase unwinds the DNA double strand and allows the use of both strands as a template. It is also the leading and the lagging strand synthesized, but not in the same way. The holoenzyme starts with the leading strand by the set of the primase primer is bound and synthesizes these continuously. Subsequently synthesized strand is much more complicated because of 5 ' → 3' and the synthesis proceeds not by the 3 ' → 5' may extend. Therefore, it is synthesized in fragments, resulting in many different 5'-3 ' synthesis. The fragments are called Okazaki fragment. A fragment is still a coiled into a loop portion of the single strand DNA, which is the active site of the α - subunit, and runs in the same direction as the leading strand. After about 1000 nucleotides of the lagging strand is released and performed the next loop in the active site, to the primase is again a primer. The resulting gaps are filled in by DNA polymerase I, as the DNA polymerase III, the repair function 5 '→ 3' is missing. In addition, the residues of RNA primers are removed.

Swell

  • Berg, Jeremy M.; Tymoczko, John L.; Stryer, Lubert: Biochemistry, Berlin / Heidelberg 2003 ISBN 3-8274-1303-6
  • Madigan, Michael T.; Martinko, M. John; Parker, Jack: Biology of Microorganisms, London 2003 ISBN 0-13-066271-2
  • Cellular component
  • DNA replication
  • Nucleotidyltransferase
  • Protein complex
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