The malate-aspartate shuttle is one of the #2# shuttle systems used by the cell for the purpose of delivering the reducing equivalents of #"NADH"# from the cytosol to the mitochondrial matrix. It is mostly active in the liver, kidney and heart cells and #2.5# molecules of #"ATP"# can be generated from the electrons being passed off in the respiratory chain.
During glycolysis , #"2 NADH"# molecules are produced per #1# molecule of glucose which all takes place in the cytosol. #"NADH"# is also produced in the Krebs cycle , along with other products not mentioned here. The thing is, #"NADH"# produced during Krebs is present in the mitochondrial matrix, whereas the #"NADH"# produced during glycolysis is present in the cytosol.
#color(white)(----)ul(color(white)(-----)"Net Yield"color(white)(----))#
#color(white)(-------)color(magenta)"Glycolysis: 2 NADH"#
#color(white)(-------)color(orange)"Krebs cycle: 6 NADH"#
Why did I mention this? Well, #"NADH dehydrogenase"#, a key enzyme in the electron transport chain (ETC), is located in the inner mitochondrial membrane. This enzyme can only accept electrons from #"NADH"# located in the matrix and NOT in the cytosol as the inner mitochondrial membrane is not permeable to #"NADH"#, meaning, the cytosolic #"NADH"# cannot pass through to get to the matrix side, and therefore, is unable to pass off its electrons in the ETC. This is where the malate-aspartate shuttle will come into play.
Overall Steps
#underbrace"OAA"_(color(blue)"cytosol")stackrel(NADH + H^(+))rarrunderbrace"malate"_(color(blue)"cytosol")+"NAD"^(+)#
#underbrace"malate"_(color(blue)"cytosol")stackrel"(malate-alphaketoglutarate transporter)"[-------->]underbrace"malate"_(color(red)"matrix")#
#underbrace"malate"_(color(red)"matrix")stackrel("NAD"^(+))rarrunderbrace"OAA"_(color(red)"matrix")+"NADH + H"^(+)#
#underbrace"OAA"_(color(red)"matrix")stackrel"glutamate"rarrunderbrace"aspartate"_(color(red)"matrix")+alpha-"ketoglutarate"#
#underbrace"aspartate"_(color(red)"matrix")stackrel"(malate-alphaketoglutarate transporter)"[-------->]underbrace"aspartate"_(color(blue)"cytosol")#
#underbrace"aspartate"_(color(blue)"cytosol")stackrel(alpha-"ketoglutarate")rarrunderbrace"OAA"_(color(blue)"cytosol")+"glutamate"#
In order for another cycle of the shuttle to start, #"OAA"# has to be regenerated in the cytosol which is why you will see #"OAA"# being converted to #"aspartate"#, which will then transport out to the cytosol and being converted back into #"OAA"#again.
