Membrane Permeability to Pyruvate?

Question

Pyruvate seems to pass easily through the outer membrane of the mitochondrion but has difficulty entering the inner membrane (and gets in by H+ symport). I have two questions: (1) what property of pyruvate disallows it from passing through the inner membrane? Is it its charge? and (2) what structural differences are there between the outer and inner membranes of the mitochondrion that create their disparate permeabilities to pyruvate?

Answer 1

Pyruvate is polar and negatively charged and can not go through any membrane. Therefore, it relies upon membrane proteins to be transverse membranes of both the outer and inner membrane.

The membrane protein called porin allow pyruvate molecules to pass through the outer membrane and the recently mitochondrial carrier protein shuttles pyruvate across the inner membrane.

Answer 2

Pyruvate is negatively charged molecule and highly polar which do not allow it to pass through any biological membrane.

So now comes the question that how it enters outer mitochondrial membrane?

The outer mitochondrial membrane contains proteins called porins, which form channels that allow the free diffusion of molecules smaller than about 6000 daltons. It is suggested that pyruvate traverses the outer mitochondrial membrane through the large, relatively non-specific, voltage-dependent anion channel (VDAC), or porin.

In contrast to outer mitochondrial membrane, Inner mitochondrial membrane is impermeable and does not have such channels. The transport of pyruvate through inner membrane occur by carrier mediated process. Recent studies suggest that two proteins, mitochondrial pyruvate carriers MPC1 and MPC2, form a hetero-oligomeric complex in the inner mitochondrial membrane to facilitate pyruvate transport (Kyle S. McCommis and Brian N. Finck, 2015).