Methylation is a common post-translational modification (PTM) of proteins, and is mostly introduced by site-specific S-adenosylmethionine (AdoMet)-dependent methyltransferase (MTase) enzymes.
The ε-amino group of a lysine side chain can accept up to three methyl groups.
RHEA:51736 describes the reaction for the first methylation.
To model this reaction, Rhea focuses on the reactive part, i.e. the L-lysine residue which is transformed into an N⁶-methyl-L-lysine residue:

Rhea identifier	Rhea name	Reactive part
RHEA-COMP:9752	L-lysyl-[protein]	CHEBI:29969 (L-lysine residue)
RHEA-COMP:13053	N6-methyl-L-lysine-[protein]	CHEBI:61929 (N6-methyl-L-lysine residue)