In the coagulation cascade, which enzyme complex is responsible for the conversion of prothrombin (Factor II) to thrombin (Factor IIa) and is composed of Factor Xa, Factor V, calcium ions, and phospholipid?

Explanation

## The Prothrombinase Complex **Key Point:** The prothrombinase complex (also called the **prothrombin-activating complex**) is the final common pathway enzyme complex that catalyzes the conversion of prothrombin (Factor II) to thrombin (Factor IIa), the most important serine protease in hemostasis. ### Composition of the Prothrombinase Complex | Component | Role | Source | |-----------|------|--------| | **Factor Xa** | Enzyme (serine protease) | Activated by both intrinsic and extrinsic pathways | | **Factor V** | Cofactor (accelerates reaction ~200,000-fold) | Released from platelets and plasma | | **Calcium ions (Ca²⁺)** | Bridging cofactor | Serum | | **Phospholipid (PL)** | Membrane scaffold | Platelet membrane (phosphatidylserine) | **High-Yield:** The prothrombinase complex is the **final common pathway** enzyme complex. Its assembly is essential for the explosive amplification of thrombin generation. ### Mnemonic: **"XVCaL"** **X** = Factor X**a**, **V** = Factor **V**, **Ca** = **Calcium**, **L** = **Lipid** (phospholipid) This complex catalyzes: **Prothrombin (II) → Thrombin (IIa)** ### Comparison with Tenase Complex | Feature | Tenase Complex | Prothrombinase Complex | |---------|---|---| | **Substrate** | Factor X | Prothrombin (Factor II) | | **Product** | Factor Xa | Thrombin (Factor IIa) | | **Enzyme** | Factor IXa (intrinsic) or Factor VIIa (extrinsic) | Factor Xa | | **Cofactor** | Factor VIIIa (intrinsic) or Tissue Factor (extrinsic) | Factor Va | | **Pathway** | Intrinsic and extrinsic | Final common pathway | | **Requires** | Ca²⁺ + PL | Ca²⁺ + PL | **Clinical Pearl:** Deficiency or dysfunction of any component of the prothrombinase complex (Factor Xa, Factor V, calcium, or phospholipid availability) results in prolonged prothrombin time (PT) and impaired thrombin generation. ### Mechanism of Thrombin Generation Factor Xa cleaves two peptide bonds in prothrombin: 1. Cleavage releases **Fragment 1+2** (contains the Gla domain) 2. Cleavage releases **Fragment 2** (contains the kringle domains) 3. Remaining molecule = **Thrombin (Factor IIa)** with its catalytic serine protease domain This is why Factor V is called a **"cofactor"** — it does not have enzymatic activity itself but dramatically accelerates the rate of prothrombin cleavage by Factor Xa.