Shuttle Involved in Cytosol-Matrix Exchange of NADH and NADPH

Reduced coenzymes such as NADH and NADPH do not permeate the inner membrane of the mitochon­drion to any significant extent.

However, reduced pyridine nucleotides are known to be produced in a number of reactions in the cytosol (the reduction of NAD⁺ at the glyceraldehyde step in glycolysis is an important example) and the re-oxidation of NADH oc­curs via the mitochondrion.

The mechanism involves a set of reactions called a shuttle (Fig. 16-32).

1. Glycerol Phosphate Shuttle:

The glycerol phosphate shuttle (Fig. 16-32) involves:

(1) Glycerol phosphate dehydrogenase in the cytosol,

(2) Glycerol phosphate dehydrogenase on the outer surface of the inner mitochondrial membrane, and

(3) The re­duction of Q in the electron transport chain.

Dihydroxyacetone phosphate, NADH, and H⁺ react in the cytosol to form glycerol phosphate, which diffuses through the outer mitochondrial membrane to the outer surface of the inner membrane. There the glyc­erol phosphate reacts with the membrane dehydro­genase to form dihydroxyacetone phosphate, which returns to the cytosol. The membrane-bound dehydro­genase employs a flavoprotein as a coenzyme, and the F_P becomes reduced during the reaction.

Subse­quently, the electrons from the reduced F_P are passed directly into the electron transport system at the Q step. Because the NAD⁺ →F_P step of electron trans­port is skipped, only two ATP are generated for each pair of electrons that enters in this fashion from the cytosol.

2. Malate-Aspartate Shuttle:

Another shuttle, the malate-aspartate shuttle, can also transport the hydrogens accepted during the re­duction of NAD ⁺ in the cytosol across the inner mem­brane (Fig. 16-32). H⁺ transported in this manner into the matrix, enters the electron transport chain via NADH dehydrogenase, and as a result three ATP are generated for each pair of electrons.

3. Total ATP Production from the Catabolism of Glucose:

The complete oxidation of a molecule of glucose by glycolysis, the TCA cycle, and electron transport is accompanied by a gross production of 38 or 40 ATP or a net production of 36 or 38 ATP. The ATPs result from substrate-level phosphorylations and electron trans­port coupled to oxidative phosphorylation. An ac­counting of ATP production is described in Figure 16- 33.

The different totals for ATP production (i.e., gross production of either 38 or 40) depend on which shuttle is used to transport H⁺ from NADH into the mito­chondrion. H ⁺ from the glycerol phosphate shuttle is accepted by F_P and as a result one coupling step is by­passed. Hydrogen from the malate-aspartate shuttle enters the electron transport chain earlier, so that all three coupling sites are utilized.