l - People

is controlled by the periplasmic permease system. In Escherichia coli one of these
consists of periplasmic binding proteins and two or three different types of membrane-
bound proteins. Upon ligand binding, the periplasmic binding protein undergoes a
conformational change and can then be recognized by the membrane-bound components
of the permease system. The interactions at the interface between the ligand-bound
protein and the membrane-bound proteins cause the ligand to be released from the
binding protein, bind to the membrane-bound protein, and subsequently be translocated
across the membrane. The glutamine transport system includes the monomeric
Glutamine binding protein (GluBP), which binds L-glutamine with a dissociation
constant (KD) of 5⋅10 -7 at 5°C.
GluBP is comprised of two domains linked by a hinge region. It resembles otherl
periplasmic binding proteins and the neurotransmitter-binding domains of ionotropic
glutamate receptors, to which it is closely related. Closure of these proteins is estimated
to take ~5 ns. Direct simulation of the hinge bending motion by Molecular Dynamics
(MD) should therefore be possible, albeit somewhat expensive. Simulations performed
by Pang et al. for that length of time failed to produce closure but resulted in an accurate
identification of the hinge at residues 88 and 183. The so-named large domain contains
both the N- and C-termini and consists of two stretches of polypeptide, residues 1-84 and
186-226. The small domain therefore consists of a single stretch of polypeptide. The
two are separated by hinges composed of two antiparallel β-strands (Figure 5), which are
located at residues 85-89 and 181-185 according to Hsiao et al, or at residues 88 and 182
according to Pang et al.
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