Processive versus distributive methy
l group transfer was assessed for pea Rubisco
large subunitmethy
ltransferase, a SET domain protein
lysine methy
ltransferase cata
lyzing the formation of trimethy
llysine-14 in the
large subunit of Rubisco. Cata
lytica
lly competent comp
lexes between an immobi
lizedform of des(methy
l) Rubisco and Rubisco
large subunit methy
ltransferase were used to demonstrate enzymere
lease that was co-incident with and dependent on formation of trimethy
llysine. Cata
lytic rate constantsdetermined for formation of trimethy
llysine were considerab
ly
lower (~10-fo
ld) than rate constantsdetermined for tota
l radio
labe
l incorporation from [
3H-
methyl]-
S-adenosy
lmethionine. Doub
le-reciproca
lve
locity p
lots under cata
lytic conditions favoring monomethy
llysine indicated a random or ordered reactionmechanism, whi
le conditions favoring trimethy
llysine suggested a hybrid ping-pong mechanism. Theseresu
lts were compared with doub
le-reciproca
l ve
locity p
lots and product ana
lyses obtained for
HsSET7/9(a monomethy
ltransferase) and
SpCLR4 (a dimethy
ltransferase) and suggest a predictive abi
lity of doub
le-reciproca
l ve
locity p
lots for sing
le versus mu
ltip
le methy
l group transfers by SET domain protein
lysinemethy
ltransferases. A mode
l is proposed for SET domain protein
lysine methy
ltransferases in which initia
lbinding of po
lypeptide substrate and
S-adenosy
lmethionine is random, with po
lypeptide binding fo
llowedby deprotonation of the
![](/images/gifchars/epsi<font color=)
lon.gif" BORDER=0 >-amine of the target
lysy
l residue and subsequent methy
lation. Fo
llowing methy
lgroup transfer,
S-adenosy
lhomocysteine and monomethy
lated po
lypeptide dissociate from monomethy
ltransferases, but di- and trimethy
ltransferases begin a successive and cata
lytica
lly ob
ligatory deprotonationof enzyme-bound methy
lated
lysy
l intermediates, which a
long with binding and re
lease of
S-adenosy
lmethionine and
S-adenosy
lhomocysteine is manifested as a hybrid ping-pong-
like reactionmechanism.