How DJ-1 protein protects cells from reactive carbonyl species

by Zhanibek Bekkhozhin

ABSTRACT

Glycolysis is an ancient metabolic pathway that allows cells to extract energy from glucose and obtain crucial building blocks. Unfortunately, one of the glycolysis steps catalyzed by triosephosphate isomerase results in an accidental generation of methylglyoxal (MG), a toxic side product that can lead to glycation of proteins and nucleic acids. Due to the presence of two adjacent carbonyl groups, methylglyoxal (MG) is extremely electrophilic and easily reacts with cellular nucleophiles, such as amine and sulfhydryl groups on macromolecules, forming hemiaminals and hemithioacetals (HTA) on the aldehyde group. This adduct formation is called glycation and it leads to altered physicochemical properties of biopolymers which can render them inactive. To prevent glycation, dedicated systems have evolved to convert free methylglyoxal (MG) into harmless lactate. However, the fate of proteins and nucleic acids that already reacted with methylglyoxal (MG) is unclear. Recently, in a series of experiments it was discovered that Parkinson associated DJ-1 protein can “repair” these transiently damaged macromolecules by breaking down early adducts of MG, which led to the classification of DJ-1 as a protein/nucleic acid deglycase. This conclusion was made using published kinetic and thermodynamic data, according to which early glycation products, such as hemithioacetals (HTA) and hemiaminals are fairly stable so that they could be purified from MG and their deglycation could be monitored after the addition of DJ-1. However, when our research group performed spectrophotometrical determination of equilibrium constant for the formation of hemithioacetals (HTA), equilibrium constant value was 100 times smaller than the previously reported one meaning that there is always free MG present in equilibrium with HTA. In addition, this equilibrium turned out to be extremely rapid. Subsequent experiments showed that at low concentration of MG and high concentration of HTA, DJ-1 works slowly, whereas at high concentrations of free MG, DJ-1 reaction rate significantly increases. Combined with additional biochemical experiments, we unequivocally concluded that DJ-1’s true substrate is MG, noGlycolysis is an ancient metabolic pathway that allows cells to extract energy from glucose and obtain crucial building blocks. Unfortunately, one of the glycolysis steps catalyzed by triosephosphate isomerase results in an accidental generation of methylglyoxal (MG), a toxic side product that can lead to glycation of proteins and nucleic acids. Due to the presence of two adjacent carbonyl groups, methylglyoxal (MG) is extremely electrophilic and easily reacts with cellular nucleophiles, such as amine and sulfhydryl groups on macromolecules, forming hemiaminals and hemithioacetals (HTA) on the aldehyde group. This adduct formation is called glycation and it leads to altered physicochemical properties of biopolymers which can render them inactive. To prevent glycation, dedicated systems have evolved to convert free methylglyoxal (MG) into harmless lactate. However, the fate of proteins and nucleic acids that already reacted with methylglyoxal (MG) is unclear. Recently, in a series of experiments it was discovered that Parkinson associated DJ-1 protein can “repair” these transiently damaged macromolecules by breaking down early adducts of MG, which led to the classification of DJ-1 as a protein/nucleic acid deglycase. This conclusion was made using published kinetic and thermodynamic data, according to which early glycation products, such as hemithioacetals (HTA) and hemiaminals are fairly stable so that they could be purified from MG and their deglycation could be monitored after the addition of DJ-1. However, when our research group performed spectrophotometrical determination of equilibrium constant for the formation of hemithioacetals (HTA), equilibrium constant value was 100 times smaller than the previously reported one meaning that there is always free MG present in equilibrium with HTA. In addition, this equilibrium turned out to be extremely rapid. Subsequent experiments showed that at low concentration of MG and high concentration of HTA, DJ-1 works slowly, whereas at high concentrations of free MG, DJ-1 reaction rate significantly increases. Combined with additional biochemical experiments, we unequivocally concluded that DJ-1’s true substrate is MG, not glycated macromolecules.