Molecular Structure of the Pyruvate Dehydrogenase Complex from Escherichia coli K-12

Abstract

The pyruvate dehydrogenase core complex from E. coli K-12, Since the pioneering experiments of Gunsalus and Hager (1) and of Koike, Reed, and Carroll (2), a-ketoacid dehydrogenase complexes from various sources have been extensively studied. Several years ago, we began to study the Escherichia coli pyruvate dehydrogenase complex from a genetic point of view (3), because it appeared that the molar ratio of the constituent polypeptide chains was far from unity. We therefore began to study the regulation of the synthesis of the enzyme complex, and it became clear during these studies (4-6) that one would be hampered in further such experiments without a precise knowledge about the composition of this multienzyme complex; thus, we turned our attention toward an elucidation of its structure (7,8,4 MATERIALS AND METHODSCells and Reagents. Pyruvate dehydrogenase complex was prepared from K-12 strain YMel and from the regulatory mutants K1-1 LR8-13 and K1-1 LR8-16, which synthesize the enzyme complex constitutively (6). Growth conditions, purification procedures, enzymc assays, and sources of reagents were described (8).Determination of Flavin-Adenine Dinucleotide (FAD). Our instrumentation was checked with FAD purchased from Boehringer iIannheim GmbH. The expected extinction coefficient (450 nm) of 1.13 cm2/mmol (9) was found after the substance was dried for 3 days under reduced pressure at 600 over P205. All spectra were measured in 0.05 M potassium phosphate, pH 7.5, with a Zeiss PMQJI spectrophotometer. Reduction and reoxidation of enzyme-bound FAD in the presence of 6.5 M urea was performed exactly as described by Massey, Hofmann, and Palmer (10). All protein concentrations for these spectral analyses were determined by the biuret reaction (11); the relation of dry weight to biuret assay has been reported$.Determination of Polypeptide Chain Ratio. Pure polypeptide chains were isolated by preparative dodecylsulfate-polyacrylamide gel electrophoresis (8). They were subjected to analytical electrophoresis under conditions essentially the same as those of Weber and Osborn (12). The gels contained 10% acrylamide, 0.135% methylene bisacrylamide, 0.05% sodium dodecylsulfate, and 0.05 M phosphate, pH 7.0. The gel columns were 60-mm high and had a diameter of 5 mm. Electrophoresis was for about 2.5 hr at 6 mA per column. The gels were stained for 2 hr with Commassie Brilliant Blue (12), and were immediately destained electrophoretically. The gels were then kept for 8 days in 7.5% acetic acid-5% methanol to remove all background stain. Stain intensity was measured with a Joyce-Loebl microdensitometer equipped with a red filter. Each column was measured twice; after the first tracing the column was rotated around its longitudinal axis by 900 and a second tracing was taken. The difference between the two measurements did not exceed 5% per band. Each point recorded in Fig. 1 is an average value of these two measurements. The areas below the tracings were cut out and weighed with a microbalance. (The density of the r...

The stoichiometry of polypeptide chains in the pyruvate dehydrogenase multienzyme complex of E. Coli determined by a simple novel method

“…These results therefore agree quite well with those of Vogel et al [7] except that we find the ratio of El/E2 approaches a value of 2 on occasion (samples 3 and 4).…”

“…The amidination method for determining chain ratios described above offers advantages of speed and simplicity compared with conventional gel scanning procedures which, to be reliable, demand careful, even laborious, calibration [7]. Since the proteins in a 000. bmol lysine/subunit mol.…”

“…There is, however, no agreement on the chain ratio of El : E2 : E3 in the native complex. A ratio of 2 : 2 : 1 is favoured by Reed and his colleagues [5,6] but it has been reported [7] that some preparations of the enzyme carry a molar excess of El compared with E2 and E3 (about 1.3 : 1 : 1). This 'excess' El can be stripped off by chromatography of the enzyme complex on columns of calcium phosphate gel, the chain ratio for the resulting 'core' complex then being 1 : 1 : 1 [7].…”

Determination of the Chain Stoichiometries from the Number of Reactive Sulfhydryl Groups in the Pyruvate Dehydrogenase Complexes of Azotobacter vinelandii and Escherichia coli

“…Without having determined directly the number of pyruvate dehydrogenase chains and knowing that the variability in the number of these chains, is between 1 and 2 [6,24,25], no choice can be made between the values of two and four lipoyl residues per chain. We arrive therefore either at a chain stoichiometry pyruvate dehydrogenase : lipoyltransacetylase : lipoamide dehydrogenase of 1.…”

Polypeptide chain stoicheiometry in the self‐assembly of the pyruvate dehydrogenase multienzyme complex of Escherichia coli

“…However, the total numbers of polypeptide chains of each type in the native enzyme are controversial. A stoicheiometry (El : E2 : E3) of 2:2: 1 is favoured by Reed and co-workers [3] but other measurements of the stoicheiometry [4,5] have indicated values of > 1: 1: 1 and we have proposed [5,6] that a value of 2: 1: 1 may represent an upper limit.…”