Table 1. Identification of reactive Gln residues in OPN.

	Reactive Gln	Peptide	ppm	Spectral counts	Conservation
15 min	42	QNLLAPQNAVSSEETNDFK	3.7	4	25
	193	AIPVAQDLNAPSDWDSR	6.5	4	16
	248	KANDESNEHSDVIDSQELSK	4.3	1	23
5 h	5	QADSGSSEEK	3.6	5	11
	15	QLYNK	0.3	1	18
	34	YPDAVATWLNPDPSQK	7.1	12	24
	36	QNLLAPQNAVSSEETNDFK	5.3	2	25
	42	QNLLAPQNAVSSEETNDFK	3.3	8	25
	36/42	QNLLAPQNAVSSEETNDFK	3.6	1	25/25
	55	QETLPSK	5.5	5	25
	164	RPDIQYPDATDEDITSHMESEELNGAYK	3.0	2	22
	193	AIPVAQDLNAPSDWDSR	8.4	9	16
	213	GKDSYETSQLDDQSAETHSHK	5.2	7	23
	217	GKDSYETSQLDDQSAETHSHK	4.9	5	4
	226	GKDSYETSQLDDQSAETHSHKQSR	3.6	1	12
	248	KANDESNEHSDVIDSQELSK	5.3	11	23

OPN was labeled with 5-(Biotinamido)pentylamine for 15 min or 5 h by TG2. Identified reactive Gln residues are underlined and shown in bold. The ppm differences between the measured and expected masses are listed. The spectral counts include all identified peptides containing the specific reactive residue. Conservation shows how many of the 25 mammalian OPN sequences used in a multiple sequence alignment that contains the specific reactive Gln residue. 5-(Biotinamido)pentylamine labeling and MS analyses were repeated four times.