Part 3 - ETC (Electron Transport Chain)

Part 3 – ETC (Electron Transport Chain)

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v NAD and FAD are used to capture H-atoms to become NADH₂ and FADH₂ . This is called oxidation.  NADH₂ and FADH₂  will donate their H-atoms in mitochondrion and become NAD and FAD again . This is called reduction. This is done for two purposes. (1) The regained NAD and FAD can be used again to keep the Glycolysis and the TCA-cycle running. (2) The H-atoms will go into the ETC (Electron Transport Chain) to generate ATP (biochemical energy) for  yeast growth and multiplication.

                     

v Hydrogen (H)

Hydrogen atom is the only chemical element without  neutrons. It contains a single positively charged  proton (H+) and a single negatively charged electron(e-). It is the lightest element and is the most abundant chemical substance in the universe. Hydrogen plays a particularly important role in oxidation and reduction reaction in wine chemistry.

                                                                                 

v ETC (Electron Transport Chain) 

ETC is a series of protein complexes embedded in the mitchondrial  inner membrane. It works as following :

NADH₂  donates the 2H to complex 1. The coenzyme flavin mononucleotide (FMN) is a stronger oxidizing agent than NAD. NADH reductase is the 2 protons (H⁺) acceptor and iron-sulfur protein (FeS) is the 2 electrons (e^– ) acceptor. The protein CoQ (ubiquinone) transports these electrons to complex 3, and the protein cyt C (cytochrome C) transports them to complex 4. Finally these electrons will be captured by the oxygen (O), which is obtained by the cellular respiration. The oxygen works here as an electron acceptor (oxidizing agent). Coupled with each electron transport, one hydrogen ion (H⁺) is pumped by the complex from the matrix to the intermembrane space . There are 2 electrons  and 3 complexes. These 6 H⁺ increase the H⁺ concentration in the intermembrane space. That means the H+ concentration in the intermembrane space  is higher than that in the matrix. This is called pH gradient. By a pH gradient, H+ will diffuse from an area of high concentration to an area of lower concentration. It is  called chemiosmosis. By this way the H+ comes back into the matrix through the ATP synthase, which can synthesize 3  ATP. The 6 returned H⁺ will then be connected with the O and the  e^–  to form water :  2 H⁺ + 2 e^- + ^1/₂ O₂ ---> H₂O, which will be reused in the TCA-cycle.

FADH₂  donates  its 2 hydrogen atoms by complex 2, Succinate dehydrogenase, which does not pump H⁺. Coupled with the electron transport, 4 H⁺  are pumped through. That’s why FADH₂ gives only 2 ATP.

v Cellular respiration

Yeast cells set off CO₂ and takes in O₂ . This is cellular respiration, with objective to transform glucose into ATPs.

v Recapitulation EMP,CAC & ETC

Wine yeasts are  aerobic and anaerobic. That means they can grow with or without oxygen. In grape juice they use sugars to grow. In the presence of oxygen they transform one sugar  in cytoplasm and mitochondia to yield 38 ATP and the growth is optimum. In the absence of oxygen the cellular respiration will  stop. Pyruvate will not go into mitochondia. In this case pyruvate will be decardoxylated and reduced to ethanol. The main purpose of this reaction is to regenerate NAD which is needed for  the glycolysis.  The net gain is 2 ATP and the growth is minimum.

  

v Oxidation & Reduction in wine chemistry

l In oxidation, molecule A is oxidized by oxidising agent NAD. It loses 2H and becomes molecule B.

    Oxidation is the loss of hydrogen.

l In reduction, molecule B is reduced by reducing agent NADH+H. It gains 2H and becomes molecule A.

    Reduction is the gain of hydrogen.

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P.S.

We have seen how and why alcohol is formed. However, alcoholic fermentation produces not only ethanol, but also several other compounds like glycerol, fatty acids, higher alcohols, esters, diacetyl, etc. Fortunately! Without them, wine would have little organoleptic interest.

Coming next month, in Part 4, we’ll see why one 6-carbon molecule of glucose, after phosphorylation, is not split into two 3-carbon molecules of  the same structure but of 2 different structures, and even stranger not in equal proportion but in a ratio of 96% to 4%.  Also we’ll see how an why glycerol is synthesized?  

Fascinating how things work out in the nature.