In the intrinsic pathway of coagulation, which factor acts as the cofactor for Factor IXa to form the tenase complex?

Explanation

## The Tenase Complex: Intrinsic Pathway **Key Point:** The tenase complex is the critical enzyme complex of the intrinsic pathway that activates Factor X to Factor Xa. It consists of Factor IXa (enzyme) + Factor VIIIa (cofactor) + calcium + phospholipid surface. ### Structure of the Tenase Complex ```mermaid flowchart LR A[Factor IX]:::outcome -->|Activated by Factor XIIa| B[Factor IXa]:::outcome C[Factor VIII]:::outcome -->|Activated by Thrombin| D[Factor VIIIa]:::outcome B -->|+ Factor VIIIa| E[Tenase Complex]:::action D -->|+ Ca²⁺ + PL| E E -->|Activates| F[Factor X → Xa]:::outcome ``` **High-Yield:** The tenase complex is also called the "intrinsic tenase" or "Factor IXa-VIIIa complex." This is the rate-limiting step of the intrinsic pathway and is where hemophilia (Factor VIII deficiency) and Christmas disease (Factor IX deficiency) exert their effects. ### Comparison: Tenase vs. Prothrombinase | Complex | Pathway | Enzyme | Cofactor | Substrate | Product | | --- | --- | --- | --- | --- | --- | | **Tenase** | Intrinsic | Factor IXa | Factor VIIIa | Factor X | Factor Xa | | **Prothrombinase** | Common | Factor Xa | Factor Va | Factor II | Factor IIa (Thrombin) | **Mnemonic:** **"8 & 9 make X"** — Factor VIII + Factor IX (as IXa) → Factor X activation. **Clinical Pearl:** Hemophilia A (Factor VIII deficiency) and Hemophilia B/Christmas disease (Factor IX deficiency) both present with prolonged aPTT because the tenase complex is defective. Both conditions have similar clinical bleeding patterns because they disrupt the same enzymatic step. **Warning:** Do NOT confuse Factor VIII with Factor V. Factor V is the cofactor for the prothrombinase complex (Factor Xa), NOT the tenase complex.