Question

What are the names of the four complexes in the electron transport chain? What molecules/electron carriers donate electrons to the complexes? What molecules carry the electrons between the complexes (e.g. from complex II to III)? The following are electrons carriers in the complexes: cytochromes a, a3, b, and c1, FAD, FMN. Assign them to the right complexes.

Answer 1

Electron transport chain (ETC) is a series of proteins and organic molecules which are located in the inner membrane of the mitochondria. In ETC electrons are passed from donor Molecule to other carrier molecule in a series of redox reactions and energy is released in form of ATP.

ETC has four electron carrier complexes as follows:

1. Complex I – It is called NADH dehydrogenase, it is composed of polypeptide chains and has several Fe-S centers.

2. Complex II: Succinate Dehydrogenase, It Transfers 2 electrons from succinate to coenzyme Q, It has 4 different protein subunits

3. Complex III: Ubiquinone Cytochrome C Oxidoreductase, which is made up of heme groups and Fe-S centers.

4. Complex IV: Cytochrome C Oxidase, It is made up of cytochromes and Fe-S centers.

ETC initiates as Complex I accepts electrons from NADH and transfers an electron. Complex II includes succinate dehydrogenase is also link between the citric acid cycle and the electron transport chain, Complexes I and II both reduced coenzyme Q,CoQH2 which is the substrate for Complex III. Complex III transfers the electrons from CoQH2 and reduces cytochrome c which is the substrate for Complex IV. Complex IV transfers the electrons from cytochrome c and reduces molecular oxygen into water.

Molecules involved in ETC:

1. NADH: It’s an electron donor and can initiate redox reaction. It transfers an electrons to complex I and forms NAD+.As electrons passes through complex I energy is released, this energy is utilized to pump protons (H+) from the matrix into the intermembrane space of mitochondria.
2. FADH: it helps to transfer electrons into the transport chain through complex II. They can accept or donate electrons
3. Coenzyme Q (CoQ): CoQ has repeating isoprene subunits, Coenzyme Q is an electron carrier when it’s in the hydrophobic bilipid layer of the mitochondria. They can be donate or accept electrons.
4. Cytochromes: They are proteins that contain heme prosthetic groups which function as one electron carriers. The heme iron is involved in one electron transfers involving the Fe2+ and Fe3+ oxidation states. Cytochromes are a, a3, b and c depending on their porphyrin structure.
5. Iron-Sulfur Proteins: These proteins which participates in ETC are always present in Fe2+ and Fe3+ oxidation states.
6. Copper Proteins Copper bound proteins: they are present in the Cu+ and Cu2+ oxidation states.

+Outer membrane ATP Synthase 5 NADH H2O NAD+H ATP citric acid cycle ADP Matrix Succinate Fumarate H2o Inner membrane Intermembrane space