Structure and Properties of the Putrescine Carbamoyltransferase of Streptococcus faecalis

Ornithine and putrescine carbamoyltransferases from Streptococcus faecalis ATCC11700 have been purified and their structural properties compared. The molecular weight of native ornithine Carbamoyltransferase, measured by moleclar sieving, is 250000. It is composed of six apparently identical subunits with a molecular weight of 39000, as determined by cross‐linking with the bifunctional reagent glutaraldehyde followed by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate. Using the same method, putrescine Carbamoyltransferase is a trimer of 140000 consisting of three identical subunits with a molecular weight of 40000. Ornithine Carbamoyltransferase displays a narrow specificity towards its substrate, ornithine. In contrast, putrescine Carbamoyltransferase carbamoylates ornithine and several diamines (diaminopropane, diaminohexane, spermine, spermidine, cadaverine) in addition to its preferred substrate, putrescine, but with a considerably lower efficiency than for putrescine. The kinetic mechanism of putrescine Carbamoyltransferase has been investigated. Initial velocity studies yield intersecting plots using either putrescine or ornithine as substrate, indicating a sequential mechanism. The patterns of protection of the enzyme by the reactants during heat inactivation as well as the results of product and dead‐end inhibition studies provide evidence for a random addition of the substrates. The putrescine inhibition that is induced by phosphate does, however, suggest that a preferred pathway exists in which carbamoylphosphate is the leading substrate. The different kinetic constants have been established. The properties of putrescine Carbamoyltransferase are compared to the known properties of other carbarnoyltransferases. The evolutionary implications of this comparison are discussed.