Vitamin K (phylloquinone) is antidotal. The structure of phylloquinone, Vitamin K, is marked by the presence of a phytyl group. Phylloquinone, the most common form of the vitamin, is directly involved in photosynthesis. Phylloquinone A is the next early electron acceptor in PSI. MK-7 may be converted from phylloquinone (K) in the colon by "Escherichia coli" bacteria. A stereoisomer of phylloquinone is called vitamin k (note the difference in capitalization). Phytomenadione is often also called phylloquinone, vitamin K, or phytonadione.