Are alpha-ketoglutarate and glutamate involved in all transamination reactions?
The major transaminases such as the aspartate transaminase and alanine transaminase involve the glutamate α-ketoglutarate interconversion. Even the GABA aminotransferase employs glutamate/α-ketoglutarate. It seems that all transaminases use glutamate/α-ketoglutarate; in fact most of them do. 

These are a few that don't:


Beta-alanine--pyruvate transaminase.
Tryptophan--phenylpyruvate transaminase. 
Pyridoxamine--pyruvate transaminase. 
Aminolevulinate aminotransferase.
Serine--pyruvate transaminase. 
Aspartate--prephenate aminotransferase. 


And a few more (see here). Most of these are present in plants.