In: Chemistry
Be able to state the purpose of the glycerol 3-phosphate and malate-aspartate shuttles. Be able to state why the glycerol 3-phosphate shuttle is considered to less efficient than the malate-aspartate shuttle in promoting ATP production?
The malate-aspartate shuttle (sometimes also the malate shuttle) is a biochemical system for translocating electrons produced during glycolysis across the semipermeable inner membrane of the mitochondrion for oxidative phosphorylation ineukaryotes. These electrons enter the electron transport chain of the mitochondria via reduction equivalents to generate ATP. The shuttle system is required because the mitochondrial inner membrane is impermeable to NADH, the primary reducing equivalent of the electron transport chain. To circumvent this, malate carries the reducing equivalents across the membrane
The glycerol-3-phosphate shuttle is a mechanism that regenerates NAD+ from NADH, a by-product ofglycolysis. Its importance in transporting reducing equivalents is secondary to the malate-aspartate shuttle.
Since the malate-aspartate shuttle regenerates NADH inside the mitochondrial matrix, it is capable of maximizing the number of ATPs produced in glycolysis (3/NADH), ultimately resulting in a net gain of 38 ATP molecules per molecule of glucose metabolized. Compare this to the glycerol 3-phosphate shuttle, which reduces FAD+ to produce FADH2, donates electrons to the quinone pool in the electron transport chain, and is capable of generating only 2 ATPs per NADH generated in glycolysis (ultimately resulting in a net gain of 36 ATPs per glucose metabolized). (These ATP numbers are prechemiosmotic, and should be reduced in light of the work of Mitchell and many others. Each NADH produced only 2.5 ATPs, and each FADH2 produces only 1.5 ATPs. Hence, the ATPs per glucose should be reduced to 32 from 38 and 30 from 36. It should also be noted that the extra H+ required to bring in the inorganic phosphate during Oxidative-Phosphorylation contributes to the 30 and 32 numbers as well).