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mechanism followed by lactose permease is follows a series of steps
that are defined as the alternating access mechanism. The sugar binds
to a proton in the periplasmic space to a transport protein. After
the binding, the sugar from the periplasmic space is exposed to the
cytoplasm at the same time when the periplasm closes up. Exposure of
the sugar to the cytoplasm is followed by the dissociation of the
proton and the sugar. The proton is lost to the cytoplasm while the
sugar moves across the membrane. The lactose permease therefore
allows the movement of sugar any time that a proton moves in a
particular direction by coupling the thermodynamically unfavorable
concentration of lactose in a cell to that of proton uptake within
1: Yin et al., 3985
1: Substrate Transport in LacY
The structure of the
lactose permease is made up of 417 amino acids and 12 transmembrane
helices, divided into two bundles, the N-domain and the C-domain.
There are bundles which has four helices that lines the hydrophobic
cavity which is open on the cytoplasmic side of the cell membrane.
(Litman & Stein, ch. 5). The transmembrane has six residues which
participates in sugar binding and that’s open to the cytoplasmic side
of the membrane. The coordinated movements of the helices enable the
alternating changes in the binding sites to allow the transport to
(LacY) is a transmembrane protein that also known as lactose
transporter allows for the passage of lactose across the phospholipid
bilayers of the cell membrane. This is a secondary active transport
and is the major facilitator superfamily membrane proteins. The
lactose permease engages in active transport of lactose by using the
energy of the electrochemical proton gradient which moves in the same
inward direction as the Lactose across the cell membrane. It is a
symporter protein, hence works in the plasma membrane where it is
responsible for allowing the absorption of lactose in the body
through the phospholipid bilayers. .