Opening Image: The C-terminal domain of Peptide:N-glycosidase F (PNGase F) from Flavobacterium meningosepticum. This domain consists of 137 amino acid residues.
It is useful to group protein folds into three broad classes, based on the predominant secondary structural elements contained within them. All-α proteins are comprised entirely of α helices. All-β proteins contain only β sheets. α/β proteins contain both. Some small proteins have little, if any, secondary structure.
The simplest all-β fold is the β sandwich in which two essentially independent sheets pack face-to-face to form a sandwich structure. The two β sheets are usually twisted. Packing the sheets at an angle with respect to one another allows efficient interdigitation of the interior side chains. The β sandwich is the most common of all protein folds in the Structural Classification of Proteins (SCOP) data base known, due in large part to its simple yet versatile design.
transparent surface model.
Pyruvate kinase. This enzyme has four identical subunits and each subunit has three domains. the seven-stranded β barrel in one of the subunits, which serves as the structural scaffold of Domain I.
The β-barrel fold is found in many proteins. A single large β sheet twists and coils to form a closed structure in which the first strand comes round to hydrogen bond to the last strand. The structure may be solid, as in the barrel shown here, or it may enclose a cavity, as seen in the next example.
Retinol binding protein. This 182-residue protein functions as a carrier for retinol (vitamin A).
The eight β-strands enclose a cavity at one end of the β barrel, creating a hydrophobic pocket for the water-insoluble retinol.
retinol, shown in lime green.
spacefilling model of complex.