Biochemistry Pathways

ELECTRON TRANSPORT

Introduction:

The electron transport chain is series of protein complexes that shuttle electrons from NADH, FADH₂, and ascorbate (synthetic) to molecular oxygen to produce water. These complexes are located in the inner membrane of the mitochondrion.

How does the cell make ATP (which holds energy in the form of an acid anhydride bonds) from a series of oxidation-reduction reactions?

Review of Electrical Work and the Nernst equation:

Physics:

Electrical work = VQ V = potential difference; Q = charge

Chemistry:

In electrochemistry VQ = E x nF

E = cell potential
n = # electrons transferred
F (Faraday) = charge of a mole of electrons
= 96,485 coulombs

Connection to thermodynamics:

G is a measure of maximum useful work that can be done.

G = - nFE (the term on the right is electrical work, see above)

For conditions other than standard, G varies:

G = G^o + RT lnQ where Q is the equilibrium expression [C][D]/[A][B]

For the equivalent expression for redox, substitute G = -nFE.

E = E^o - RT/nF lnQ or E = Eo -0.059/n logQ

this is the Nernst equation

Example:

Pyruvate + 2 H⁺ + 2 e^- --> lactate E^o' = - 0.19 V = left
NAD+ + H⁺ + 2 e^- --> NADH E^o' = - 0.32 V = right = reduction

ER - EL = - 0.32 -(- 0.19) = -0.13 V must reverse equations because reactions are not spontaneous as reductions in this order!

The chemical sense of this is that NAD+ can't be reduced by lactate --> pyruvate, but the reaction can occur in the other direction.

Components of the electron transport system:

A. Flavoproteins (proteins with FMN or FADH² tightly associated)
B. Iron-sulfur proteins
C. Quinones
D. Heme proteins

Flavoproteins:

Flavoproteins are classed as such because they contain either flavin adenine dinucleotide (FADH) or flavin mononucleotide (FMN),usually tightly bound to their protein. These proteins participate in a variety of redox reactions; those listed below are ass ociated with electron transport.

NADH-Q reductase, also known as NADH dehydrogenase, is composed of 25 polypeptides

NADH --> 2 e- --> FMN --> FMNH2 --> FeS proteins (see below)

Succinate-Q reductase, also known as succinate dehydrogenase, contains 4 polypeptides.

Iron-sulfur proteins:

Iron-sulfur proteins, formerly called non-heme iron proteins, contain iron that is associated with the protein via inorganic sulfides and the sulfhydryl groups of cysteine, rather than through the protophorphyrin IX structure.

Iron sulfur proteins are placed into one of three classes based on the organization of the iron ion, inorganic sulfides, and cysteine side chains.

Quinones:

Ubiquinone has a highly nonpolar side chain and is mobile in the mitochondrial membrane; it carries electrons from complexes I & II to complex III.

Cytochromes:

Remainder of electron carriers are cytochromes, in which an iron ion is bound by protophorphyrin IX.

Iron sulfur proteins tend to be located predominantly at the highly reduced (NADH) end of the electron transport chain, but from ubiquinone to O2, all electron carriers except one are cytochromes.

It is noteworthy that the heme proteins and FeS proteins all transfer one electron at a time (contrasted to NADH, FADH₂ and Q, which carry 2 electrons.

Use of inhibitors to identify sequence of electron transport:

When you study any biochemical process, it is useful to have inhibitors.

By using inhibitors, we can determine the chemical consequences of blocking a particular step in process; genetics is also useful in this regard.

At the time that the nature and sequence of electron transport reactions was being worked out, there were no UV-Vis spectrophotometers, only hand-held spectroscopes.

After testing numerous chemicals that were known to affect respiration, biochemists found that they fell into three classes, or acted at three sites. From these studies and a variety of others, the sequence of reactions involved in electron transport was determined.

Shuttle systems for the mitochondrion:

Since the mitochondrial inner membrane must be impervious to protons (see Oxidative phosphorylation), it is not surprising that other, larger charged molecules can't move across this membrane without specific transport proteins. Indeed, the intact mitochondrion is not permeable to NAD⁺ or NADH.

In the brain, transportation of reducing equivalents into and out of mitochondria occurs via a glycerol 3-P shuttle.

mito

                  _________________________________________
                                                 E-FAD    E-FADH2       
                             Glycerol 3-P  ------------->      DHAP        	inner membrane
                  _________________________________________

                             Glycerol 3-P  <-------------      DHAP              	cytosol
                                                 NAD+    NADH + H+

Net reaction:

NADH_cyto + H⁺ + E-FAD_mito ---> NAD+_{cyto_{+ E-FADH_2mito}}

The energy consequences of this shuttle: cytoplasmic NADH yields only 2 ATP's/mole vs. 3 ATP for NADH formed inside the mitochondrion.

The heart and liver represent special cases, they have a malate-aspartate shuttle:

                               NAD+     NADH             Glu     a-KG

                 Malate    -------------->       OAA        ------------>      aspartate             mito

     _______________________________________________________________
     _______________________________________________________________


                 Malate    <--------------       OAA        <------------      aspartate              cyto

                               NAD+     NADH                Glu     a-KG

This shuttle is readily reversible, therefore no energy is expended (NADH = 3 ATP).