This review is focused on three types of enzymes decarboxylating very different substrates: (1) Thiamin diphosphate (ThDP)-dependent enzymes reacting with 2-oxo acids; (2) Pyridoxal phosphate (PLP)-dependent enzymes reacting with α-amino acids; and (3) An enzyme with no known co-factors, orotidine 5'-monophosphate decarboxylase (OMPDC). While the first two classes have been much studied for many years, during the past decade studies of both classes have revealed novel mechanistic insight challenging accepted understanding. The enzyme OMPDC has posed a challenge to the enzymologist attempting to explain a 1017-fold rate acceleration in the absence of cofactors or even metal ions. A comparison of the available evidence on the three types of decarboxylases underlines some common features and more differences. The field of decarboxylases remains an interesting and challenging one for the mechanistic enzymologist notwithstanding the large amount of information already available.
Keywords: 2-oxo acids; catalysis; circular dichroism; decarboxylation; orotidine 5'-monophosphate; pyridoxal 5-phosphate; thiamin diphosphate; α-amino acids.