Liver 3‐Phosphoglycerate Kinase

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Abstract
Homogeneous phosphoglycerate kinase from bovine liver possesses a maximum ultraviolet absorption at 278 nm (A1%, 1cm280= 6.7; Amax/Amin= 2.26; ɛ280= 31.5 mM−1× cm−1). The enzyme consists of about 420 amino‐acid residues and is a slightly acidic protein with an isoelectric point of 6.5 as expected from amino‐acid analysis. The most notable features of the chemical composition are two tryptophan, 12 methionine and four half‐cystine residues per enzyme molecule. Although phosphoglycerate kinases from mammalian tissues are partially similar to each other, clear differences in serine, glutamic acid, glycine, cysteine, valine, leucine, tyrosine, tryptophan and arginine contents were found. Fingerprinting and column chromatography of tryptic digests of the S‐carboxymethylated protein confirm the data of amino‐acid analysis.Liver phosphoglycerate kinase is inactivated when modified with either p‐chloromercuribenzoate or 5,5′‐dithio‐bis(2‐nitrobenzoic acid) (Nbs2). The enzyme has two thiol groups available for reaction with Nbs2 under denaturing conditions, one of which is essential for catalysis. After reduction by NaBH4 four cysteine residues per molecule were determined with Nbs2 suggesting the presence of a disulfide bridge.Using sedimentation equilibrium studies, the molecular weight was found to be 49600. Gel filtration yielded values of 43000–50000. By analytical dodecylsulfate‐polyacrylamide gel electrophoresis a molecular weight of 45600 was estimated. Inconsistent with these results is the value 37500 obtained by thin‐layer gel chromatography in 6 M guanidine‐HCl. Sedimentation velocity experiments revealed a sedimentation coefficient s20,w, = 3.4 S. The Stokes radius was 2.77 nm, the partial specific volume v̄ 0.747 ml × g−1. The diffusion coefficient was found to be 76.9 μm2× s−1 by analytical gel filtration. From these data a molecular weight of 44000 was calculated. Other physical constants of bovine‐liver phosphoglycerate kinase are: frictional ratio f/f0= 1.18, axial ratio = 3.3, maximal degree of hydration = 0.1 g per g of protein.Bovine‐liver phosphoglycerate kinase could not be dissociated into smaller subunits by treatments which have caused dissociation of various other proteins (8 M urea, 6 M guanidine‐HCl, dodecyl sulfate, carboxymethylation, maleylation). All experiments strongly support the lack of subunit structure of the enzyme.Some characteristics of bovine‐liver phosphoglycerate kinase are compared with the corresponding proteins from rabbit muscle, yeast and human erythrocytes.