Page 221 - Color Atlas of Biochemistry

P. 221

212 Organelles

Transport systems B. Malate and glycerophosphate shuttles
Two systems known as “shuttles” are avail-
Mitochondria are surrounded by an inner and
an outer membrane (see p. 210). The outer able to allow the import of reducing equiva-
lents that arise from glycolysis in the cyto-
membrane contains porins, which allow +
smaller molecules up to 10 kDa in size to plasm in the form of NADH+H . There is no
transporter in the inner membrane for
pass. By contrast, the inner membrane is also +
impermeable to small molecules (with the NADH+H itself.
In the malate shuttle (left)—which operates
exception of water and the gases O 2 ,CO 2 ,
and NH 3 ). All of the other substrates of mito- in the heart, liver, and kidneys, for exam-
chondrial metabolism, as well as its products, ple—oxaloacetic acid is reduced to malate by
malate dehydrogenase (MDH, [2a]) with the
therefore have to be moved through the inner
+
help of NADH+H . In antiport for 2-oxogluta-
membrane with the help of special transport- rate, malate is transferred to the matrix,
ers.
where the mitochondrial isoenzyme for
MDH [2b] regenerates oxaloacetic acid and
+
A. Transport systems NADH+H . The latter is reoxidized by complex
I of the respiratory chain, while oxaloacetic
The transport systems of the inner mitochon-
drial membrane use various mechanisms. acid, for which a transporter is not available in
the inner membrane, is first transaminated to
Metabolites or ions can be transported alone
(uniport, U), together with a second substance aspartate by aspartate aminotransferase (AST,
[3a]). Aspartate leaves the matrix again, and
(symport, S), or in exchange for another mol-
ecule (antiport, A). Active transport—i. e., in the cytoplasm once again supplies oxalo-
acetate for step [2a] and glutamate for return
transport coupled to ATP hydrolysis—does transport into the matrix [3b]. On balance,
not play an important role in mitochondria.
+
only NADH+H is moved from the cytoplasm
The driving force is usually the proton gra- into the matrix; ATP is not needed for this.
dient across theinnermembrane(blue star)
The glycerophosphate shuttle (right) was
or the general membrane potential (red star; discovered in insect muscle, but is also active
see p. 126).
The pyruvate (left) formed by glycolysis in in the skeletal musculature and brain in
+
the cytoplasm is imported into the matrix in higher animals. In this shuttle, NADH+H
formed in the cytoplasm is used to reduce
–
–
antiport with OH .The OH ions react in the
+
intermembrane space with the H ions abun- glycerone 3-phosphate, an intermediate of
glycolysis (see p. 150) to glycerol 3-phos-
dantly present there to form H 2 O. This main- phate. Via porins, this enters the intermem-
–
tains a concentration gradient of OH .The
–
import of phosphate (H 2 PO 4 )is drivenin a brane space and is oxidized again there on the
exterior side of the inner membrane by the
similar way. The exchange of the ATP formed
in the mitochondrion for ADP via an adenine flavin enzyme glycerol 3-phosphate dehydro-
genase back into glycerone 3-phosphate. The
nucleotide translocase (center) is also depen- reducing equivalents are passed on to the
+
dent on the H gradient. ATP with a quadruple
negative charge is exchanged for ADP with a respiratory chain via ubiquinone (coenzyme
Q).
triple negative charge, so that overall one
The carnitine shuttle for transporting acyl
+
negative charge is transported into the H - residues into the mitochondrial matrix is dis-
rich intermembrane space. The import of ma- cussed on p. 164.
late by the tricarboxylate transporter,which is
important for the malate shuttle (see B)is
coupled to the export of citrate, with a net
export of onenegative chargeto the exterior
again. In the opposite direction, malate can
leave the matrix in antiport for phosphate.
When Ca 2+ is imported (right), the metal cat-
ion follows the membrane potential. An elec-
+
troneutral antiport for two H or two Na +
serves for Ca 2+ export.

216 217 218 219 220 221 222 223 224 225 226