1. A mol.wt. of 40030 +/- 830 has been estimated for phosphoglycerate kinase in concentrations less than 0.1 g/100 cm3 comparing favourably with expected values from X-ray diffraction measurements by 10% lower than the previously reported molecular weights made at higher concentrations. 2. The so20w, was estimated to be 3.12(+/-0.02)x10(-13)s and the coefficient had a low concentration dependency giving a g value (concentration-dependency) of 2.3 +/- 1.6cm3 .g-1. This agrees with previous qualitative observations. 3. By using fluctuation-intensity spectroscopy, the D20,w was estimated to be 7.4(+/-0.2)x10(-11)m2.s-1, and this was indistinguishable from the D20,w calculated from ultracentrifuge results. The water of hydration was estimated to be 0.46 g/g of protein. 4. It is inferred from the estimates that phosphoglycerate kinase associates with an interaction coefficient at 20 degrees C for monomer/dimer of between 10 and 12 cm3.g-1. 5. The ratio of molecular asymmetry (a/b) was estimated to be 2.5+/-0.2 from the values of D20,w and water of hydration. This compares favourably with the ratio from the overall dimensions estimated from X-ray diffraction measurements.

1. Previous work showed that yeast phosphoglycerate mutase (EC 2.7.5.3) has a mol.wt. of between 107000 and 110000. Preliminary examination showed that at dilutions less than 0.1 g/1 the enzyme dissociated into its subunits. 2. This dissociation was quantitatively examined by both equilibrium and velocity centrifugation. 3. The mathematical analysis of the equilibrium records was tested against oxyhaemoglobin in a variety of ionic strengths and at two temperatures. 4. The estimated L2,4 (interaction coefficient) for oxyhaemoglobin generally agreed with published values except at 6 degrees C in 0.9 M-NaCl, when it was 2.5 times larger than the published value. 5. Statistical analysis of ultracentrifugal-equilibrium experiments showed that the predominant reaction for phosphoglycerate mutase was monomer in equilibrium tetramer, to give an L1,4 of 40.3+/-23.4 (S.D.)1(3)-g(-3) at 20 degrees C. Decreasing the temperature decreased the association to given an enthalpy of between 40 and 60kJ/mol. 6. Analysis of velocity experiments carried out with concentrations varying from 0.3 to 17 g/1 gave an L1,4 of 3111(3)-g(-3). Incorporating errors from estimating S20,w into the analysis showed that this estimate could range from 893 to 1421(3)-g(-3). 7. The concentration-dependence of S20,w was 0.95 litre-g-1 and s020,w for the tetramer was 66.9ps. 8. These results are discussed in relation to the activity of the enzyme.

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