Question

17. Why are fewer protons pumped into the intermembrane space when FADH, is oxidized than when NADH is oxidized via the mitochondrial electron transport system? (2 marks) 18. NADH-Q oxidoreductase has many a-helices in the membrane portion. What properties of a-helices allow them to be buried in membranes? (2 marks) 19. The pyruvate dehydrogenase complex of E. coli has a weight of roughly 4.6 MDa. If we assume that only the protein contributes significantly to the weight, approximately how many amino acid residues are in the complex? (3 marks)

Answer 1

Q17

Ans:

NADH donates electrons in redox reactions because its electrons are at a high energy level. Hence NADH transfer electrons directly to Complex I and rapidly becomes NAD+. This flow of electrons to Complex I in a series of redox reactions leads to the release of energy and complex I then, in turn, pump the proton from the matrix into the intermembrane space.

In contrary to NADH, electrons of FADH2 are at a lower energy level. Hence the electrons are not transferred to complex I directly, instead the electrons are fed into the transport chain via complex II which does not pump protons across the membrane into intermembrane space.

Because of these reasons fewer protons are pumped into intermembrane space when a molecule of FADH2 are oxidized compared to NADH.

Q18

Ans:

The alpha helices proteins which span the intermembrane space are known to posses hydrophobic patch of amino acids. These hydrophobic side chains then form van der Waals interactions with the fatty acyl chains of the membrane and thus shields the polar carbonyl (C=O) and imino (NH) groups of the peptide bond, which are all hydrogen-bonded to one another.

OR

In other way transmembrane proteins are amphipathic, i.e. they have both hydrophobic and hydrophilic patch or region. The hydrophobic regions pass through the membrane and interact with the hydrophobic tails of the lipid molecules in the interior of the bilayer, where they are sequestered away from water. Whereas the hydrophilic regions are exposed to water on either side of the membrane.

Q19

Ans:

The average weight of amino acid is 110Da.

Now the weight of the PDH complex is given as 4.6MDa (4.6 Mega Dalton, 1MDa = 10^6 Da)

4.6 x 10^6 MDa = 46 x 10^5 Da.

Hence

1 amino acid = 110 Da

        X           = 46 x 10^5 Da.

by cross mulptiplying (X on left side is the number of amino acid);

110X = 46 x 10^5

      X = 46 x 10^5/110

       X = 4.18 x 10^4

Therefore total number of amino acids in PDH can be 4.18 x 10^4 (i.e. 41,800 amino acid)