Differential scanning calorimetry of cytoplasmic aspartate transaminase

Differential scanning calorimetry has been applied to study factors affecting the thermally induced denaturation of cytoplasmic aspartate aminotransferase, a dimeric pyridoxal enzyme. The consequences of binding of coenzyme and substrate derivatives to both the apo and holo forms of the enzyme were investigated and are interpreted in terms of the stabilization of the native form of the enzyme. The binding of pyridoxal phosphate coenzyme increases the thermal stability of the apoenzyme by approximately 27 kcal mol-1 as judged by the change in free energy differences between the native and denatured states of the protein. The stabilization produced by coenzyme binding to the apoprotein appears to be primarily due to the Schiff's base and phosphoryl moieties of the coenzyme; association of the pyridine ring component is without significant structural consequence. Pyridoxal phosphate binding to the subunits of the dimer occurs in a noncooperative fashion as judged by the appearance of transitions unique to the apo, holo, and intermediate enzyme forms in a calorimetric titration. Holoenzyme stability depends on the chemical nature of the catalytically significant group occupying the C-4' position of the bound coenzyme. The stabilization afforded by binding of the aldehyde form (pyridoxal phosphate) which exists as an internal Schiff's base with Lys 258 is diminished when this bond is chemically reduced or when the aldehyde is replaced by an amine (pyridoxamine phosphate). Apoenzyme is also shown to be stabilized by the presence of substrates in the absence of coenzyme. The differential scanning calorimetry results thus confirm previous findings derived from nuclear magnetic resonance studies on the ability of apoenzyme to bind substrates (Martinez-Carrion, M. Cheng, S., and Relimpio, A. (1973) J. Biol. Chem. 248, 2153-2160). Substrates and their analogues perturb the holoenzyme stability and the order of increasing influence on the pyridoxal form of the holoenzyme is aspartate, erythro-hydroxyaspartate, alpha-ketoglutarate, and alpha-methylaspartate. While all these compounds form stable binary enzyme-substrate complexes (Jenkins, W.T., and D'Ari, L. (1966) J. Biol. Chem. 541, 5667-5674), the complex with alpha-methylaspartate produces anomalous changes in the protein structure which are reflected in the calorimetric parameters. This suggests that caution be exercised in the use of analogues as substrate substitutes in crystallographic work. Differential scanning calorimetry also appears as a sensitive method with which to study the stereochemical dependence of ligand binding on enzyme-induced thermal stabilization. This is illustrated by the use of 4-carbon dicarboxylic acids where only those in the conformation favorable for binding are effective in stabilizing the holoenzyme.