Plant and animal PLDs have a consistent molecular structure, characterized by sites of catalysis surrounded by an assortment of regulatory sequences. The active site of PLDs consists of four highly conserved amino acid sequences (I-IV), of which motifs II and IV are particularly conserved. These structural domains contain the distinguishing catalytic sequence "HxKxxxxD" (HKD), where H, K, and D are the amino acids histidine (H), lysine (K), aspartic acid (D), while x represents nonconservative amino acids. These two HKD motifs confer hydrolytic activity to PLD, and are critical for its enzymatic activity both "in vitro" and "in vivo". Hydrolysis of the phosphodiester bond occurs when these HKD sequences are in the correct proximity.