Sandbox 666: Difference between revisions

''Eco''RI is composed of two homodimers, so it has two identical subunits (Representation of one <scene name='Sandbox_666/Monomer_structure/4'>subunit</scene>) of 31 kDa, but it is possible to have homotetramers at high concentrations. The constitutive monomers are 276 amino acids long. ''Eco''RI and all the other restriction enzymes show a common structural core, which is a  α/β domain. The constitutive subunits of ''Eco''RI are organized into a single α/β domain (five strands <scene name='Sandbox_666/B_sheet/1'>β</scene> sheet, which is surrounded  by <scene name='Sandbox_666/Helix/1'>two α helices</scene> ). Four of these five β strands are parallel whereas the fourth (β4) is in an anti-parallel orientation to the others.<ref name="B">Refinement of ''Eco''RI endonuclease crystal structure: a revised protein chain tracing.

In the old model the N-terminal section of each subunit forms the inner arm which wraps around the DNA molecule (The arm brings the DNA molecule to the catalytic cleft.).The new chain tracing, based on new elements of electron density and a new interpretation that alters the assignment of specific amino acid residues to some of the original features<ref name="B" />. In the new model,the Inner arm is the extended chain motif (Met¹³⁷to Ala¹⁴²)is a segment of extended polypeptide chain that runs through the major groove of the DNA, roughly parallel to the DNA backbone<ref name="B" />. The outer arm is composed of two minor β strands linked together by a loop (the outer arm is 14 amino acids long, four of these amino acids belong to the loop).