The beta chain of mRNA capping enzyme has triphosphatase activity. The function of the capping enzyme also depends on the guanylyltransferase activity conferred by the alpha chain (see Pfam:PF01331)

1: Isolation and characterization of the Candida albicans gene for mRNA 5'-triphosphatase: association of mRNA 5'-triphosphatase and mRNA 5'-guanylyltransferase activities is essential for the function of mRNA 5'-capping enzyme in vivo. Yamada-Okabe T, Mio T, Matsui M, Kashima Y, Arisawa M, Yamada-Okabe H; FEBS Lett 1998;435:49-54. PMID:9755857

2: Structure and mechanism of yeast RNA triphosphatase: an essential component of the mRNA capping apparatus. Lima CD, Wang LK, Shuman S; Cell. 1999;99:533-543. PMID:10589681