label: 3-O-[alpha-D-xylosyl-(1->3)-beta-D-glucosyl]-L-seryl-[EGF-like domain protein] + UDP-alpha-D-xylose <=> 3-O-[alpha-D-xylosyl-(1->3)-alpha-D-xylosyl-(1->3)-beta-D-glucosyl]-L-seryl-[EGF-like domain protein] + UDP + H(+)
label_lc: 3-o-[alpha-d-xylosyl-(1->3)-beta-d-glucosyl]-l-seryl-[egf-like domain protein] + udp-alpha-d-xylose <=> 3-o-[alpha-d-xylosyl-(1->3)-alpha-d-xylosyl-(1->3)-beta-d-glucosyl]-l-seryl-[egf-like domain protein] + udp + h(+)