The maltose transporter maltoporin (Mr 49,913) spans the outer membrane of E. coli. It is comprised of 18 successive antiparallel Beta – strands arranged like the staves of a barrel to create a channel through the membrane. Beta -turns connect the strands on the periplasmic side of the membrane; loops connect the strands on the outer side. The N – and C – termini coincide with the ends of Beta – strands and are both on the periplasmic side.

A. Calculate the average number of residues per outside loop?

B. Given that the distance between C(alpha) of neighboring Beta – strands is 0.5 nm, what are the circumference and diameter of the Beta – barrel formed by the polypeptide backbone? Of course much of the interior of the barrel is occupied by side chains that narrow the channel and specify passage of only maltose and maltodextrins.

C. Given that the width and thickness of maltose are about 0.5 nm, what percent of the diameter would the channel for maltose transport need to occupy?