請問上述圖中，哪一個酵素是關鍵酵素 (key enzyme)？ (2 分)

2. \( \sigma_{ij} = \begin{bmatrix} \sigma_{xx} & \tau_{xy} & \tau_{xz} \\ \tau_{yx} & \sigma_{yy} & \tau_{yz} \\ \tau_{zx} & \tau_{zy} & \sigma_{zz} \end{bmatrix} = \begin{bmatrix} 3 & 0 & -2 \\ 0 & 2 & 0 \\ -2 & 0 & 0 \end{bmatrix} \) is a stress tensor, find the principal stresses and the unit vectors of principal directions by using eigenvalues and eigenvectors. (20%)