Which of the following enzyme is present on outer membrane of mitochondria ?

Question

Accepted Answer

The correct option is 2: Monoamine oxidase

The correct option is 2: Monoamine oxidase (MAO) is the classic textbook example of an enzyme uniquely and exclusively situated on the outer membrane of the mitochondria (OMM). Its location is structurally and functionally critical. MAO is responsible for catalyzing the oxidative deamination of monoamines, which include important neurotransmitters like serotonin, dopamine, and norepinephrine, as well as pharmacologically active amines. By anchoring itself to the outer membrane, MAO is strategically positioned to regulate the concentration of these monoamines within the cytoplasm, preventing them from entering the inner compartments of the mitochondrion or modulating their levels before they are packaged into synaptic vesicles. The enzyme exists in two forms, MAO-A and MAO-B, but both are flavoproteins embedded in the OMM, making it the definitive marker enzyme for this specific cellular compartment.

The incorrect option is 1: Carnitine fatty acid acyl transferase

This general term refers to a system crucial for fatty acid transport, encompassing Carnitine Palmitoyltransferase I (CPT I) and CPT II. While CPT I is indeed located on the outer mitochondrial membrane (OMM) where it converts fatty acyl-CoA to acylcarnitine for transport, the overall carnitine shuttle involves CPT II which resides on the inner mitochondrial membrane (IMM). Since the question asks for an enzyme present on the OMM, the general term for the transferase system may be considered imprecise because its full operation spans both membranes. Monoamine oxidase, unlike the CPT system, is a protein whose entire biological function and structure are specifically anchored to the OMM, making it a more specific and singular choice.

The incorrect option is 3: $\beta$-oxidation enzyme

$\beta$-oxidation is the metabolic pathway responsible for breaking down fatty acids into acetyl-CoA, which then feeds into the Krebs cycle. The spiral of enzymes that perform this breakdown—such as Acyl-CoA dehydrogenase, Enoyl-CoA hydratase, and Thiolase—are located inside the **mitochondrial matrix**. The matrix is the innermost compartment of the mitochondrion and is the primary location for most of the core energy-releasing reactions that fuel the cell. Therefore, the enzymes of $\beta$-oxidation are contained deep within the organelle and are not associated with the outer membrane.

The incorrect option is 4: Citrate synthetase

Citrate synthetase (or Citrate synthase) is a critical enzyme that catalyzes the first reaction of the **Citric Acid Cycle (Krebs cycle)**. This entire metabolic cycle operates exclusively within the **mitochondrial matrix**. Its function is to condense acetyl-CoA and oxaloacetate to produce citrate, driving the subsequent reactions that generate electron carriers. Since all components and reactions of the Citric Acid Cycle are confined to the matrix, Citrate synthetase is strictly a matrix enzyme and is not present on the outer mitochondrial membrane.