Since the discovery of the first pectin biosynthetic enzyme and its associated family in 2006, the biochemical characterization of members of this family has continued to be an active area of study. The initial study that identified GALACTURONOSYLTRANSFERASE 1 (GAUT1) as a Homogalacturonan:Galacturonosyltransferase (HG:GalAT) was facilitated by its ability to be heterologously expressed in, and purified from, the eukaryotic Human Embryonic Kidney (HEK293) cell system. This thesis identifies four additional GAUTs that can be expressed in this system and demonstrates that each is an HG:GalAT. We found that, in addition to having HG:GalAT activity, these GAUTs exhibit unique specific activities and abilities to synthesize HG products with distinguishable sizes in vitro. It was also discovered that in addition to the previously characterized GAUT1:GAUT7 complex, select GAUTs can both initiate HG biosynthesis and utilize exogenously supplemented HG acceptors for their activity. The characterization of the initiation products from one of these members demonstrated that the nucleotide sugar remains attached to the product in vitro, a previously unknown characteristic of these products. In addition to the biochemical data presented in this thesis, and a comparison of their published mutants and evolutionary history, new hypotheses are put forth for GAUT in vivo functions within the plant and for the significance of these activities for overall plant growth and development.