Glycerol phosphate shuttle

The glycerol - 3-phosphate shuttle system is a biochemical transport mechanism in eukaryotes, which serves to convey the glycolysis occurring reducing agent in the form of nicotinamide adenine dinucleotide (NADH) from the cytosol into the mitochondrial matrix. There, the electrons are transferred to ubiquinone, and thus fed to the respiratory chain where it is used to generate adenosine triphosphate ( ATP). NAD is regenerated by and is available for further metabolic processes available again.

This shuttle runs much faster than the malate - aspartate shuttle and is therefore used primarily in muscle cells and in the brain where the energy must be available quickly.

Background

In the cytosol of eukaryotic cells oxidations take place through catabolic processes, such as glycolysis. It released electrons are transferred to NAD so that NADH / H is produced. Even in the wake of the citric acid cycle, these reducing equivalents are formed, the citric acid cycle occurs in the matrix of the mitochondrion. Further, the electron energy of the reduction equivalents are introduced into the respiratory chain and eventually transferred to oxygen ( aerobic respiration ).

The inner membrane of the mitochondria is NADH or NAD is not permeable ( permeable ), nor for ATP, ADP and protons. Hereby, there would be two problems: First, the NADH formed in the cytosol can not diffuse into the matrix of the mitochondrion in order there to feed their electrons into the respiratory chain. Since this NADH is regenerated to NAD , and NAD could not get out of the matrix into the cytosol in order to attend catabolic processes to participate further. In the example of glycolysis, this would quickly grind to a halt, because the amount of NAD is limited in the cytosol.

Two shuttle process but ensures that the data stored in NADH electrons are transported into the mitochondria, and this is regenerated to NAD . The only way to balance the reducing equivalents between the cytosol and mitochondria is possible. One of the shuttle is the glycerol-3 -phosphate shuttle system and is shown below.

Occurrence

The shuttle system is found in animals, fungi and plants.

Mechanism

In the cytosol is reduced to glycerol -3-phosphate Dihydroxyacetone phosphate ( DHAP ). It is NADH / H is oxidized to NAD . This reaction is catalyzed by a soluble cytosolic NAD -dependent glycerol-3 -phosphate dehydrogenase ( cGPD, EC 1.1.1.8 ) catalyzes. In a second step the glycerol -3-phosphate is oxidized back into dihydroxyacetone phosphate in the mitochondrial membrane. The case re- released electrons and protons are transferred to enzyme-bound flavin adenine dinucleotide (FAD ), which is reduced to FADH2. This reaction is catalyzed a membrane-bound FAD -dependent mitochondrial glycerol-3 -phosphate dehydrogenase ( MgPd, EC 1.1.5.3 ). This FADH2 is reoxidized to FAD whereby this ubiquinone the inner mitochondrial membrane is reduced. The latter gives the electrons to the complex III of the respiratory chain.

Be the complex I of the respiratory chain is passed over for this shuttle so that formed in the balance sheet per FADH2 only 1.5 units ATP (with NADH / H is 2.5 ). When using this transport system in contrast to the malate - aspartate shuttle the energy yield from the complete oxidation of a molecule of glucose is therefore somewhat lower, it is instead an average of 32 ATP 30 ATP.