Reaction catalyzed by (E3-independent) E2 ubiquitin-conjugating enzyme (2.3.2.24)

	S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine	+		N6-(ubiquitin)n-[histone H2A]-L-lysine	=		[E1 ubiquitin-activating enzyme]-L-cysteine	+		N6-(ubiquitin)(n+1)-[histone H2A]-L-lysine
	S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine	+		N6-(ubiquitin)n-[histone H2A]-L-lysine	=		[E1 ubiquitin-activating enzyme]-L-cysteine	+		N6-(ubiquitin)(n+1)-[histone H2A]-L-lysine