These are unified by the modified structure of their connector arm, ligand-binding and dimerization pattern.
The connector arm is variable in structure and length and assumes a wide range of conformations ranging from coils to structured elements in different versions of the fold.
This clade is unified by the presence of a small, often α-helical insert, in the "connector arm" between the fourth and fifth strands of the domain.
In the molybdopterin-dependent oxidoreductase superfamily, the barrel formed by the pre-strand-3 and the connector arm regions provides an open face for accommodating the molybdopterin ligand.
The acquisition of this additional strand has resulted in the emergence of a connector arm that joins it to the terminal conserved strand of the core sheet.
The 5-stranded assemblage is unified by the addition of the fifth strand to the core sheet and the consequent emergence of the "connector arm" linking the additional strand to the terminal strand.
While the two tandem repeats are very similar in structure, they are highly divergent in sequence, and contribute different sets of residues in the connector arm to contact the metallopterin.
The unifying inserts of this clade typically occur in the region prior to strand 3 and in the region associated with the connector arm or the additional strand of the 5-stranded core.
Another previously unknown variant is seen in the N-terminal domain of the aldehyde oxidoreductases (AOR-N) (PDB: 1AOR [ 65]), wherein the connector arm assumes an extended conformation and packs as an additional strand at the fringe of the β-sheet adjacent to the strand-4 which is a specific feature of the 5-stranded versions.
For the sake of convention, we refer to the exposed surface of the core β-sheet as the "exposed face" and the opposite surface of the sheet which might be obscured by the packing helical segment, the lateral shelf or flange, and the connector arm (in the 5-stranded versions) as the "obscured face".
In the middle domain of molybdopterin-dependent oxidoreductases (PDB: 1SOX [ 64], chain A) there is an additional insert of 2 β-strands associated with the connector arm, which results in an even more complex 3-layered structure, with the two inserts forming a barrel-like element within it.
