Centre of K-Space
There isn't one! K-space is a 2D plot of spatial frequencies, and so there is no positional relationship between points in k-space and points on the image. Every single point in k-space contributes to the entire image. To understand this properly, take the first k-space tutorial on this site, and be more familiar with what the Fourier transform does. Briefly: the Fourier transform relates k-space to the MRI image you know (and love!).
Remember that the data in the k-space array are spatial frequencies, not intensity values with (x, y) coordinates in real space. We can plot the k-space data like we plot the array of intensity values of an MRI image, to give us something to see. But it's not an image in the usual sense.
Further reading on this topic:
Books: MRI From Picture to Proton p52-53, MRI The Basics p43, Q&A in MRI p90