The document describes the malate-aspartate shuttle, a system crucial for transferring reducing equivalents like NADH across the inner mitochondrial membrane to facilitate ATP production. It details the process involving key enzymes and metabolic pathways in organs like the liver, kidney, and heart, which are highly active metabolically. The shuttle not only ensures efficient transport of electrons but also maintains substrate balance between the cytosol and mitochondria.

1. BY-
AAYUSHI RAMBIA
M.Sc. SEM I
DEPARTMENT OF MICROBIOLOGY
REG. NO. 18308001
PONDICHERRY UNIVERSITY

2.  A shuttle system in general sense is something that goes
back and forth in order to transfer things between the two
places.
 In biological systems a shuttle system is used to transfer
reducing equivalents across the membrane , more
specifically across the inner mitochondrial membrane.
 A shuttle system is required because there are certain
reducing equivalents such as NADH which cannot cross
the membrane, but it can reduce another molecule that
can cross the membrane, so that its electrons can reach
the electron transport chain , which is the ultimate goal to
produce ATP.

3.  The malate aspartate shuttle is required to transport the
reducing equivalent NADH , produced during glycolysis
in the cytosol into the mitochondria as the mitochondrial
inner membrane is impermeable to NADH. The rate of
transfer through this shuttle is 10-folds higher than any
other shutlle systems
 NADH is required in the TCA cycle which operates in the
mitochondria , from where the electrons are finally
accepted by oxygen in the ETC to produce ATP , the final
energy quotient.

4.  The shuttle operates in the mitochondria found mainly in
the liver , kidney and heart because these three organs
are the most metabolically active organs in the body.
 The shuttle involves four major enzymes which operate
to help transfer NADH into the mitochondria :
1) Malate dehydrogenase
2) Aspartate amino transferase
3) Malate – alphaketoglutarate antiporter
4) Glutamate aspartate antiporter

7.  The glycolytic pathway produces two molecules of NADH
in the payoff phase :
2Glyceraldehyde-3-phosphate2− + 2Pi
2− + 2NAD+ → 2(1,3-
Bisphosphoglycerate4− )+ 2NADH + 2H+
 The NADH thus formed is used to reduced oxaloacetate
present in the cytosol into malate catalysed by the
enzyme “malate dehydrogensae”.
Oxaloacetate + NADH + H+ → L-malate + NAD+

8.  The malate so formed can cross the mitochondrial inner
membrane and enter the mitochondria. This is facilitated
by the enzyme “malate-α-ketoglutarate carrier”
 The malate-α-ketogluatarate carrier helps the entry of
malate inside the mitonchomdria in exhange with a
molecule of α-ketoglutarate to the cytosol.
(CYTOSOL) (MITOCHONDRIAL MATRIX)
malate-α-ketoglutarate
L-Malate L-Malate
carrier
α-ketoglutarate α-ketoglutarte

9.  Inside the mitochondrial matrix the malate is oxidised by
NAD+ back to oxaloacetate and NADH catalysed by the
enzyme “malate dehydrogenase” (mitochondrial malate
dehydrogenase) .
Malate + NAD+ → Oxaloacetate + NADH + H+

10.  The oxaloacetate so formed needs to be transported
back to the cytosol but the mitochondrial inner membrane
is impermeable to oxaloacetate , so the oxaloacetate in
the matrix is converted into aspartate catalysed by the
enzyme “aspartate aminotransferase”.
Oxaloacetate → Aspartate
 Simulataneously glutamate is converted into α-
ketoglutarate.

11.  The aspartate so formed is tranported out of the
mitochondria into the cytosol via the “glutamate-aspartate
carrier” in exchange for cytosolic glutamate .
(CYTOSOL) (MITOCHONDRIAL MATRIX)
aspartate aspartate
L-glutamate L-glutamate

12.  Upon reaching the cytosol , the aspartate is converted
back to oxaloacetate by the enzyme “aspartate
aminotransferase” ,thus replenishing the cytosolic
oxaloacetate .
Aspartate → oxaloacetate
 Simultaneously α-ketoglutarate in the cytosol is
converted into glutamate.
 This step ensures that there is not net movement of
substrates between the cytosol and the mitochondria and
there is only the movement of electron.

13.  There are two antiporter proteins located in the inner
membrane of the mitochondria, the glutamate-aspartate
transporter (transporter I) and the malate-α-ketoglutarate
transporter (transporter II). In transporter I, the efflux of
aspartate from the mitochondria is accompanied by the
stoichiometric entry of glutamate and proton into the
mitochondria. Therefore, this electrogenically driven
process is irreversible and the rate-controlling step of the
M-A shuttle.
 The proton gradient created in the cytosol as well as the
mitochondrial matrix drives the entry and exit of
substrates via the antiporters.

14.  Principles of Biochemistry-Lehninger edition 4th
 Biochemistry by Voet and Voet
 The Metabolic Significance of the Malate-Aspartate Cycle
in Heart. Circulation Research is published by the
American Heart Association.
 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2572303
 https://www.researchgate.net/figure/The-malate-
aspartate-shuttle-is-the-principal-mechanism-for-the-
movement-of-reducing_fig3_236857315

15. THANK YOU