What are electron transport chains made up of?

Answer 1

The majority of the energy conserved during catabolism reactions occurs near the end of the metabolic series of reactions in the electron transport chain. The electron transport or respiratory chain gets its name from the fact electrons are transported to meet up with oxygen from respiration at the end of the chain. The overall electron chain transport reaction is:

2 H+ + 2 e+ + 1/2 O2 ---> H2O + energy

Notice that 2 hydrogen ions, 2 electrons, and an oxygen molecule react to form as a product water with energy released in an exothermic reaction. This relatively straight forward reaction actually requires eight or more steps. The energy released is coupled with the formation of three ATP molecules per every use of the electron transport chain.

Link to a complete animated version of Electron Transport - Brooks-Cole

Link to: Rodney Boyer Animation of Electron Transport

Pre-Initiation of Electron Transport Chain:

The electron transport chain is initiated by the reaction of an organic metabolite (intermediate in metabolic reactions) with the coenzyme NAD+ (nicotinamide adenine dinucleotide). This is an oxidation reaction where 2 hydrogen atoms (or 2 hydrogen ions and 2 electrons) are removed from the organic metabolite. (The organic metabolites are usually from the citric acid cycle and the oxidation of fatty acids--details in following pages.) The reaction can be represented simply where M = any metabolite.

MH2 + NAD+ -----> NADH + H+ + M: + energy

One hydrogen is removed with 2 electrons as a hydride ion (H-) while the other is removed as the positive ion (H+). Usually the metabolite is some type of alcohol which is oxidized to a ketone.

NAD+ is a coenzyme containing the B-vitamin, nicotinamide, shown on a previous page.

The purpose of the other seven steps in the electron transport chain is threefold:

1) to pass along 2H+ ions and 2e- to eventually react with oxygen;

2) to conserve energy by forming three ATP's; and

3) to regenerate the coenzymes back to their original form as oxidizing agents.Quiz: What are the initial reactants which start the electron transport chain?Answer2 hydrogen ionsand 2 electronsstart the chain.Oxygen andADP are alsoreactants.What are the final products of the chain?Answer3 ATP and awater moleculeare products.

Click for larger image

Initiation of Electron Transport Chain:

Once the NADH has been made from a metabolite in the citric acid cycle inside of the mitochondria, it interacts with the first complex 1 enzyme, known as NADH reductase. This complex 1 contains a coenzyme flavin mononucleotide (FMN) which is similar to FAD.

The sequence of events is that the NADH, plus another hydrogen ion enter the enzyme complex and pass along the 2 hydrogen ions, ultimately to an interspace in the mitochondria. These hydrogen ions, acting as a pump, are utilized by ATP synthetase to produce an ATP for every two hydrogen ions produced. Three complexes (1, 3, 4) act in this manner to produce 2 hydrogen ions each, and thus will produce 3 ATP for every use of the complete electron transport chain.

In addition, NADH passes along 2 electrons to first FMN, then to an iron-sulfur protein (FeS), and finally to coenzyme Q. The net effect of these reactions are to regenerate coenzyme NAD+. This regeneration of reactants occurs in many of the reactions so that a cycling effect occurs. The NAD+ is ready to react further with metabolites in the citric acid cycle.

Coenzyme Q, which also picks up an additional 2 hydrogen ions to make CoQH2, is soluble in the lipid membrane and can move through the membrane to come into contact with enzyme complex 3.

In summary, the very first enzyme complex in the electron transport chain is coupled with the formation of ATP. The coupled reaction may be written as:

a) MH2 + NAD+ ---> NADH + H+ + M + energy

b) ADP + P + energy ---> ATP + H2O