A classical enzyme active center motif lacks catalytic competence until modulated electrostatically

The cysteine proteinase superfamily is a source of natural structural variants of value in the investigation of mechanism. It has long been considered axiomatic that catalytic competence of these enzymes mirrors the generation of the ubiquitous catalytic site imidazolium-thiolate ion pair. We here report definitive evidence from kinetic studies supported by electrostatic potential calculations, however, that at least for some of these enzymes the ion pair state which provides the nucleophilic and acid-base chemistry is essentially fully developed at low pH where the enzymes are inactive. Catalytic competence requires an additional protonic dissociation with a common pKa value close to 4 possibly from the Glu50 cluster to control ion pair geometry. The pH dependence of the second-order rate constant (k) for the reactions of the catalytic site thiol groups with 4,4'-dipyrimidyl disulfide is shown to provide the pKa values for the formation and deprotonation of the (Cys)-S-/(His)-Im+H ion pair state. Analogous study of the reactions with 2,2'-dipyridyl disulfide reveals other kinetically influential ionizations, and all of these pKa values are compared with those observed in the pH dependence of kcat/Km for the catalyzed hydrolysis of N-acetylphenylalanylglycine 4-nitroanilide. The discrepancy between the pKa value for ion pair formation and the common pKa value close to 4 related to generation of catalytic activity is particularly marked for ficin (pKa 2.49 +/- 0.02) and caricain (pKa 2.88 +/- 0.02) but exists also for papain (pKa 3.32 +/- 0.01).

Evidence for conformational changes in Escherichia coli porphobilinogen deaminase during stepwise pyrrole chain elongation monitored by increased reactivity of cysteine-134 to alkylation by N-ethylmaleimide

Porphobilinogen deaminase from Escherichia coli becomes progressively more susceptible to inactivation by the thiophilic reagent N-ethylmaleimide (NEM) as the catalytic cycle proceeds through the enzyme-intermediate complexes ES, ES2, ES3, and ES4. Site-directed mutagenesis of potentially reactive cysteines has been used to identify cysteine-134 as the key residue that becomes modified by the reagent and leads to inactivation. Since cysteine-134 is buried at the interface between domains 2 and 3 of the E. coli deaminase molecule, the observations suggest that a stepwise conformational change occurs between these domains during each stage of tetrapyrrole assembly. Interestingly, mutation of the invariant active-site cysteine-242 to serine leads to an enzyme with up to a third of the catalytic activity found in the wild-type enzyme. Electrospray mass spectrometry indicates that serine can substitute for cysteine as the dipyrromethane cofactor attachment site.

Fatty acid composition of phospholipids from platelets and erythrocytes in multiple sclerosis

The fatty acid composition of the phospholipids of red blood cells and blood platelets has been investigated in multiple sclerosis patients and in normal individuals. The variation in the platelet phospholipid fatty acid pattern in normals has been measured for the first time and has been shown to be small. The relative level of linoleate, expressed as a percentage of the five main fatty acids, was found to be significantly lower in the multiple sclerosis patients than in healthy individuals, both in the red cells and platelets. A highly significant correlation was found between serum linoleate and both platelet and red cell linoleate.