Table 2 Hypothesized functions of the classified subfamilies of UPOs based on the roles of amino acid residues present in the motifs

I

FXD

Phe is basically involved in stacking interactions with other aromatic side-chains and the Asp is frequently involved in salt-bridges interacting with positively charged amino acids to create stabilizing H-bonds which can be important for protein’s stability.

may actively involve in interacting with aromatic residues and in forming stable H-bonds imparting to the structural stability.

Cys-Cys

disulfide bond is mostly involved in providing stability to protein structure.

II

RGN

Arg is frequently involved in making salt-bridges with the negatively charged amino acids creating stable H-bonds which may be crucial for the structure stability; the Gly provides the conformational stability, and the Asn is involved as protein’s active and binding sites.

may potentially interact with the hydrophobic ligands such as lipids and may show specificity for some polar substrates.

IDG

Ile in the IDG motif is involved in recognizing hydrophobic ligands; Asp forms stable H-bonds with positively charged amino acids required for protein’s stability, and the Gly again may provide conformational stability.

TXXXXXXR

Thr is often found in protein centers and capable of forming H-bonds with the polar substrates.

III

G [ML]G

the Gly provides the conformational stability, Met and Leu plays a role in binding and recognition of hydrophobic ligands.

may play important role in substrate specificity/recognition and capable of forming strong H-bonds with the polar substrates.

IV

CDA, FXXXDG, GAAXXXYE, and HXXF

Ala is involved in substrate recognition and specificity; Tyr makes stacking interactions with the aromatic side chains; His is involved in protein metal binding sites; and Phe also makes stacking interactions with aromatic side chains.

may show large interactions with the aromatic substrates and these motifs are perhaps involved in substrate recognition and binding.

V

EDXXH

His is most commonly involved in active and binding sites especially in metal binding sites and the Asp and Glu residues create the stable H-bonds.

may play an important role in reacting with positively charged amino acids.

GXG

Gly provides the conformational stability