Proposed structure for the noncovalently associated heme prosthetic group of dissimilatory nitrite reductases. Identification of substituents.

The substituents of the noncovalently associated heme prosthetic group of the bacterial nitrite reductase-cytochrome oxidase (EC 1.9.6.1 or EC 1.9.3.2.) from Pseudomonas aeruginosa (ATCC 19429) and Paracoccus denitrificans (ATCC 13456) have been identified. This was accomplished by 1H NMR, infrared, visible, and mass spectroscopic techniques applied to semi-purified heme and purified methyl ester derivatives of the iron-free porphyrin. The main structural features are as follows. 1) The macrocycle is a reduced porphyrin of the chlorin type, that is, one of the pyrrole rings has been saturated so that the beta-carbons have sp3-hybridization. 2) The chlorin is a tetracarboxylic acid. 3) The substituents of the saturated pyrrole ring are two methyl groups and two hydroxymethyl groups. 4) The substituents of the unsaturated pyrrole rings are: (i) two methyl groups (ii) a propionic acid (iii) an acrylic acid, and (iv) two directly bonded formic acid groups. No firm evidence has been obtained for the relative positions of these unsaturated beta-pyrrolic substituents around the macrocyclic ring, but a proposed structure will be discussed. Several previously inexplicable chemical properties of the heme in extracts or in the intact enzyme can be now interpreted in view of the proposed structure. The trivial name "acrylochlorin" is suggested for the macrocycle.