Components of the Respiratory Chain

1. Substrate dehydrogenises (nicotinamide nucleotide dehydrogenises)

Biological oxidations are catalyzed by enzymes which function in conjunction with coenzymes or electron carries. The nicotinamide nucleotides are coenzymes for dehydrogenises (enzymes). They are also referred to as the pyridine nucleotides because nicotinamide is a substituted pyridine.
The substrate dehydrogenises contain either nicotinamide adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate (NADP), a phosphorylated form of NAD.

2. Flavoproteins

Both complexes I and II contain dehydrogenases. Complex I contains NADH dehydrogenase while complex II contains succinic deyhdrogenase. These dehydrogenases contain flavoproteins as the main redox component, and also one or more iron sulphur (Fe-S) proteins.
NADH dehydrogenase. This enzyme is also called NADH: CoQ reductase and is located in complex I. It consists of flavorprotein, which has flavin mononucleotide (FMN) as its prosthetic group, iron-sulphur (Fe-S) centres.
Succine dehydrogenase is found in complex II. It contains covalently bound FAD as the prosthetic group and two Fe-S centres.

3. Iron-sulphur proteins (Fe-S)

Some enzymes use metallic ions linked to the Apo enzyme for transfer of electrons in oxidation-reduction reactions. Such enzymes are called metalloproteinase. The commonly involved ions are those of iron and copper. Proteins that bind iron atoms in a lattice of sulphur atoms are called non-heme iron proteins (NHI) or Iron-sulphur proteins (Fe-S).

4. Conenzyme Q (CoQ) or ubiquinone (UQ)

Coenzyme Q is an electron carrier which is strictly speaking not an enzyme. It is a lipid-soluble quinone acting as a coenzyme. Structurally it has a benzoquinone nucleus attached to an unsaturated isoprenoid side chain. The number of isoprenoid units varies from 6 to 10(CoQ6-10). Bacteria typically contain 6 isoprenoid units, and this number is also found in some yeasts. Mannalian mitochondria contain 10units. Coenzyme Q is able to diffuse rapidly in the mitochondrial membrane. This property is due to the isoprenoid tail which makes coenzymes Q highly nonpolar. Thus CoQ functions as a mobile electron narrier between the flavoproteins and the cytochromes.



Plastoquinone, Which occurs in chloroplast and functions in photosynthesis, is closely related to coenzyme Q. It contains two CH3 groups which replace the methoxyl (-OCH3) groups of coenzyme Q. In some bacterial respiratory systems menaquinone takes the place of coenzyme Q. The polysioprenoid side chain is also present in vitamins E and K.