Complex IV of mitochondrial ETS contains:

Question

Accepted Answer

The correct option is 1: Cyto a, a3 and two copper centres

Complex IV, officially known as Cytochrome c Oxidase, is the final enzyme complex in the mitochondrial Electron Transport System (ETS). Its crucial role is to catalyze the final step of aerobic respiration: the four-electron reduction of molecular oxygen ($O_2$) to water ($H_2O$), using electrons donated from reduced Cytochrome $c$. The defining components of this large enzyme are two distinct heme groups, Cytochrome $a$ and Cytochrome $a_3$, along with two copper centers, denoted as $Cu_A$ and $Cu_B$. Cytochrome $a$ and the $Cu_A$ center work together to receive electrons from Cytochrome $c$. Subsequently, these electrons are shuttled to the active site, which is formed by the heme group of Cytochrome $a_3$ and the $Cu_B$ center. This $a_3$-$Cu_B$ site is where oxygen binds and is reduced to water, a process that is vital for preventing the formation of highly destructive free radicals. The energy released by this electron transfer is also coupled to pump protons from the mitochondrial matrix into the intermembrane space, thereby maintaining the electrochemical gradient essential for ATP synthesis.

The incorrect option is 2: Cyto a2, b and Iron

This combination is incorrect because Cytochrome $a_2$ is not a standard or currently accepted designation for a component in the mitochondrial ETS. Furthermore, Cytochrome $b$ is a definitive and critical component of Complex III (Cytochrome $bc_1$ complex), which precedes Complex IV in the electron flow pathway, but it is not found within Complex IV itself. While iron is chemically present in Complex IV as a part of the $a$ and $a_3$ heme groups, listing "Iron" generally is not specific enough to define the complex, and the inclusion of Cyto $b$ makes the option factually wrong in the context of Complex IV composition.

The incorrect option is 3: Cyto b1, a2 and FAD

This option is incorrect primarily due to the presence of FAD and the non-standard cytochrome designations. FAD (Flavin Adenine Dinucleotide) is a key prosthetic group for electron carriers found in the initial complexes of the ETS, specifically Complex I (NADH Dehydrogenase) and, most notably, Complex II (Succinate Dehydrogenase). FAD is not a structural or functional component of Complex IV. Additionally, as noted before, Cytochrome $b_1$ and $a_2$ are not the correct names for the established electron-carrying molecules within this final complex, whose components are precisely identified as Cytochromes $a$ and $a_3$.

The incorrect option is 4: FAD, FAN and Cyto a1

This option incorrectly lists FAD (Flavin Adenine Dinucleotide), which, as explained, belongs to Complexes I and II, not Complex IV. The component FAN is not a recognized or standard cofactor or prosthetic group within any of the mitochondrial electron transport chain complexes, making this part of the option unsubstantiated. Lastly, while the cytochromes $a$ and $a_3$ are the correct components, Cytochrome $a_1$ is an obsolete or non-standard term, and therefore, the entire group does not represent the correct molecular composition of the Cytochrome c Oxidase complex.