Studies on the allosteric regulation of tryptophan oxygenase structure and function

Abstract

Listen

Translate article

Tryptophan oxygenase activity in mammalian and microbial systems is subject to several interesting types of regulation. These include control of its rate of biogenesis and degradation which determine the in vivo level of enzyme protein; furthermore, an allosteric regulation exists which modulates the catalytic efficiency of these enzyme molecules. 1. (1)|In the mammal, the level of hepatic tryptophan oxygenase is subject to both hormonal and substrate induction. The underlying molecular processes however differ. Glucocorticoid administration leads to an enhanced rate of enzyme synthesis, whereas high in vivo tryptophan levels convert the enzyme to holoenzyme which is more slowly degraded in vivo. Thus, treatment with either of these inducers results by a distinct mechanism in a rise in the concentration of the enzyme protein. 2. (2)|Tryptophan oxygenase is induced to high levels in Pseudomonas acidovorans by culture in tryptophan-rich media. This process involves the feedback induction of tryptophan oxygenase by its metabolite, kynurenine. 3. (3)|The catalytic efficiency of existing molecules of tryptophan oxygenase is subject to an additional regulatory process which is intrinsic within the enzyme molecule. This allosteric regulation has been studied on homogeneous preparations of tryptophan oxygenase derived from induced Pseudomonas acidovorans. This allosteric behavior is characterized by the following attributes: 3.1. (a)|The catalytic activity is a sigmoidal function of the tryptophan concentration. 3.2. (b)|The effector compound, α-methyltryptophan, which at the concentration employed is neither a substrate nor an inhibitor of this enzyme, eliminates the sigmoidicity and decreases the apparent K m for tryptophan. 3.3. (c)|The K m of the enzyme for its other substrate, oxygen, diminishes as the enzyme becomes saturated with either l-tryptophan or α-methyl-tryptophan. 3.4. (d)|The equilibrium binding of various ligands to the heme prosthetic group of tryptophan oxygenase was evaluated spectrophotometrically. The affinity of the enzyme for carbon monoxide and cyanide augments in the presence of tryptophan. 4. (4)|The concentration of sodium dodecylsulfate required to dissociate the enzyme from a tetramer into monomeric units is elevated in the presence of α-methyltryptophan. Combination of this effector compound with the allosteric site apparently augments the subunit interactions which are responsible for maintaining the quaternary integrity of the enzyme. These various studies indicate that tryptophan oxygenase contains distinct catalytic and allosteric sites. Tryptophan may combine with either, whereas α-methyltryptophan combines with high affinity at the allosteric site. Effector combination at the allosteric site results in increased affinity of the enzyme's heme for its physiological ligand, oxygen, or for other ligands such as carbon monoxide and cyanide. Furthermore, marked intensification of quaternary interactions accompanies saturation of the allosteric site.