Page 229 - Color Atlas of Biochemistry
P. 229
220 Organelles
Transport proteins C. Aquaporin-1
Aquaporins help water to pass through bio-
Illustrations B–D show transporters whose
structure has been determined experimen- logical membranes. They form hydrophilic
pores that allow H 2 O molecules, but not hy-
tally or established on analogy with other
known structures. They all belong to group drated ions or larger molecules, to pass
through. Aquaporins are particularly impor-
III of the α-helical transmembrane proteins
(see p. 214). tant in the kidney, where they promote the
reuptake of water (see p. 328). Aquaporin-2
in the renal collecting ducts is regulated by
A. Transport mechanisms antidiuretic hormone (ADH, vasopressin),
which via cAMP leads to shifting of the chan-
Some cells couple the “pure” transport forms nels from the ER into the plasma membrane.
discussed on p. 218—i. e., passive transport (1) Aquaporin-1, shown here, occurs in the
and active transport (2)—and use this mech- proximal tubule and in Henle’s loop. It con-
anism to take up metabolites. In secondary
active transport (3), which is used for exam- tains eight transmembrane helices with dif-
ferent lengths and orientations. The yellow-
ple by epithelial cells in the small intestine
and kidney to take up glucose and amino colored residues form a narrowing that only
acids, there is a symport (S) located on the H 2 O molecules can overcome.
luminal side of the membrane, which takes up
+
the metabolite M together with an Na ion. D. Sarcoplasmic Ca 2+ pump
+
+
An ATP-dependent Na transporter (Na /K +
ATPase; see p. 350) on the other side keeps Transport ATPases transport cations—they are
the intracellular Na+ concentration low and “ionpumps.” ATPasesof the Ftype—e. g., mito-
+
chondrial ATP synthase (see p. 142)—use H
thus indirectly drives the uptake of M. Finally,
a uniport (U) releases M into the blood. transport for ATP synthesis. Enzymes of the V
type, using up ATP, “pump” protons into lyso-
somes and other acidic cell compartments (see
B. Glucose transporter Glut-1 p. 234). Ptype transport ATPases are particu-
larly numerous. These are ATP-driven cation
The glucose transporters (Glut) are a family of
related membrane proteins with varying dis- transporters that undergo covalent phosphor-
ylation during the transport cycle.
tribution in the organs. Glut-1 and Glut-3
The Ca 2+ ATPase shown also belongs to the
have a relatively high af nity for glucose (K d P type. In muscle, its task is to pump the Ca 2+
= 1 mM). They occur in nearly all cells, and released into the cytoplasm to trigger muscle
ensure continuous glucose uptake. Glut-2 is
found in the liver and pancreas. This form has contraction back into the sarcoplasmic retic-
ulum (SR; see p. 334). The molecule (1)con-
alower af nity (K d = 15–20 mM). The rate of
glucoseuptakebyGlut-2istherefore strongly sistsofa singlepeptide chainthat is folded into
dependent on the blood glucose level (nor- various domains. In the transmembrane part,
which is formed by numerous α-helices, there
mally 4–8 mM). Transport by Glut-4 (K d = 2+
5 mM), which is mainly expressed in muscle are binding sites for two Ca ions (blue) ATP
is bound to the cytoplasmic N domain (green).
and fat cells, is controlled by insulin, which Four different stages can be distinguished
increases the number of transporters on the
cell surface (see p. 388). Glut-5 mediates sec- in the enzyme’s catalytic cycle (2). First, bind-
ing of ATP to the N domain leads to the uptake
ondary active resorption of glucose in the in- 2+
testines and kidney (see A). of two Ca into the transmembrane part (a).
Phosphorylation of an aspartate residue in the
Glut-1 consists of a single peptide chain
that spans the membrane with 12 α-helices Pdomain (b) and dissociation of ADP then
of different lengths. The glucose is bound by causes a conformation change that releases
2+
ions into the SR (c). Finally, dephos-
the Ca
the peptide loops that project on each side of
the membrane. phorylation of the aspartate residue restores
the initial conditions (d).
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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