Transaminases swap the amino group between an amino acid and an alpha-keto acid, and every one of them depends on a single B-vitamin cofactor: Vitamin B6.
In its active state, B6 (pyridoxine) is converted to pyridoxal phosphate (PLP). PLP sits in the enzyme active site and reacts with the incoming amino acid to form a Schiff-base aldimine; the amino group is then handed over via a pyridoxamine phosphate intermediate to the recipient keto acid. This ping-pong mechanism is the defining feature of aminotransferases such as AST and ALT.
The other vitamins serve different chemistries: thiamine (B1) as TPP drives decarboxylations, riboflavin (B2) as FAD and niacin (B3) as NAD/NADH handle redox transfers. None of these can perform amino-group transfer.
\[\boxed{\text{Transamination} \Rightarrow \text{Pyridoxal phosphate (Vitamin B6)}}\]