Nucleosome structure
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Nucleosomes are the basic building blocks of chromatin fibers. A nucleosome consists of a core containing an octamer of histone proteins and a DNA molecule 146 bp long wound around this core in two complete turns. The histone protein octamer includes four types of proteins: H2A, H2B, H3 y H4. Histone proteins are organized in dimers so:
- Two H3-H4 dimers
- Two H2A-H2B dimers
- Whole octamer
- DNA molecule wound in two complete turns around octamer. Some manganese ions complete the whole structure.
The more widespread secondary structure in histonic proteins is alfa helix.
If we situate the different types of amino acid residues in protein skeleton we can see that negatively charged residues and positively charged residues are arranged so positively charged residues are in peripherycal positions, where they can stablish ionic interations with phosphate groups on DNA molecule. Such electric charges distributions stabilizes the whole structure.
References
This page is based on 1aoi file from Proteopedia.
1aoi is a 10 chain structure with sequence from Xenopus laevis. The July 2000 RCSB PDB Molecule of the Month feature on Nucleosome by David S. Goodsell is 10.2210/rcsb_pdb/mom_2000_7. Full crystallographic information is available from OCA. For a guided tour on the structure components use FirstGlance.
