HindIII

HindIII restriction endonuclease
Crystallographic structure of the HindIII restriction endonuclease dimer (cyan and green) complexed with double helical DNA (brown) based on the PDB: 2E52​ coordinates.
Identifiers
Symbol	RE_Hindiii
Pfam	PF09518
InterPro	IPR019043
Available protein structures: Pfam structures PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

HindIII (pronounced "Hin D Three") is a type II site-specific deoxyribonuclease restriction enzyme isolated from Haemophilus influenzae that cleaves the DNA palindromic sequence AAGCTT in the presence of the cofactor Mg²⁺ via hydrolysis.

The cleavage of this sequence between the AA's results in 5' overhangs on the DNA called sticky ends:

5'-A |A G C T T-3'

3'-T T C G A| A-5'

Restriction endonucleases are used as defense mechanisms in prokaryotic organisms in the restriction modification system. Their primary function is to protect the host genome against invasion by foreign DNA, primarily bacteriophage DNA. There is also evidence that suggests the restriction enzymes may act alongside modification enzymes as selfish elements, or may be involved in genetic recombination and transposition.

The structure of HindIII is complex, and consists of a homodimer. Like other type II restriction endonucleases, it is believed to contain a common structural core comprising four β-sheets and a single α-helix. Each subunit contains 300 amino acids and the predicted molecular mass is 34,950 Da. Despite the importance of this enzyme in molecular biology and DNA technology, little information is available concerning the mechanism of DNA recognition and phosphodiester bond cleavage. However, it is believed that HindIII utilizes a common mechanism of recognition and catalysis of DNA found in other type II enzymes such as EcoRI, BamHI, and BglII. These enzymes contain the amino acid sequence motif PD-(D/E)XK to coordinate Mg²⁺, a cation required to cleave DNA in most type II restriction endonucleases. The cofactor Mg²⁺ is believed to bind water molecules and carry them to the catalytic sites of the enzymes, among other cations. Unlike most documented type II restriction endonucleases, HindIII is unique in that it has little to no catalytic activity when Mg²⁺ is substituted for other cofactors, such as Mn²⁺.