matrix in \frac{}{}

Hi, I'm trying to get this running, but it won't ... how can I have a matrix in the following formula: \begin{equation} \alpha^{\star}=\frac{\mbox{\boldmath$z$}^{T}\mbox{\boldmath$\Lambda$} \mbox{\boldmath$\Sigma$}}{\mbox{\boldmath$z$}^{T}\mbox{\boldmath$\Lambda$} \begin{bmatrix}-\mbox{\boldmath$K$}_{1}^{-1}&0\\ 0&\mbox{\boldmath$K$}_{2}^{-1} \end{bmatrix}\mbox{\boldmath$\Lambda$}^{T}\mbox{\boldmath$z$} +r\mbox{\boldmath$z$}^{T}\mbox{\boldmath$\Lambda$} \mbox{\boldmath$\Sigma$}\mbox{\boldmath$\Lambda$}^{T}{\mbox{\boldmath$z$}} \end{equation} thanks! -bodo.