Protein misfolding is a hallmark of a number of metabolic diseases, in which fatty acid oxidation defects are included. The latter result from genetic deficiencies in transport proteins and enzymes of the mitochondrial β-oxidation, and milder disease conditions frequently result from conformational destabilization and decreased enzymatic function of the affected proteins. Small molecules which have the ability to raise the functional levels of the affected protein above a certain disease threshold are thus valuable tools for effective drug design. In this work we have investigated the effect of mitochondrial cofactors and metabolites as potential stabilizers in two β-oxidation acyl-CoA dehydrogenases: short chain acyl-CoA dehydrogenase and the medium chain acyl-CoA dehydrogenase as well as glutaryl-CoA dehydrogenase, which is involved in lysine and tryptophan metabolism. We found that near physiological concentrations (low micromolar) of FAD resulted in a spectacular enhancement of the thermal stabilities of these enzymes and prevented enzymatic activity loss during a 1h incubation at 40°C. A clear effect of the respective substrate, which was additive to that of the FAD effect, was also observed for short- and medium-chain acyl-CoA dehydrogenase but not for glutaryl-CoA dehydrogenase. In conclusion, riboflavin may be beneficial during feverish crises in patients with short- and medium-chain acyl-CoA dehydrogenase as well as in glutaryl-CoA dehydrogenase deficiencies, and treatment with substrate analogs to butyryl- and octanoyl-CoAs could theoretically enhance enzyme activity for some enzyme proteins with inherited folding difficulties.
ETHE1 is an iron-containing protein from the metallo β-lactamase family involved in the mitochondrial sulfide oxidation pathway. Mutations in ETHE1 causing loss of function result in sulfide toxicity and in the rare fatal disease Ethylmalonic Encephalopathy (EE). Frequently mutations resulting in depletion of ETHE1 in patient cells are due to severe structural and folding defects. However, some ETHE1 mutations yield nearly normal protein levels and in these cases disease mechanism was suspected to lie in compromised catalytic activity. To address this issue and to elicit how ETHE1 dysfunction results in EE, we have investigated two such pathological mutations, ETHE1-p.Arg163Gln and p.Arg163Trp. In addition, we report a number of benchmark properties of wild type human ETHE1, including for the first time the redox properties of the mononuclear iron centre. We show that loss of function in these variants results from a combination of decreased protein stability and activity. Although structural assessment revealed that the protein fold is not perturbed by mutations, both variants have decreased thermal stabilities and higher proteolytic susceptibilities. ETHE1 wild type and variants bind 1±0.2 mol iron/protein and no zinc; however, the variants exhibited only ≈10% of wild-type catalytically activity. Analysis of the redox properties of ETHE1 mononuclear iron centre revealed that the variants have lowered reduction potentials with respect to that of the wild type. This illustrates how point mutation-induced loss of function may arise via very discrete subtle conformational effects on the protein fold and active site chemistry, without extensive disruption of the protein structure or protein-cofactor association.
Riboflavin, or vitamin B2, plays an important role in the cell as biological precursor of FAD and FMN, two important flavin cofactors which are essential for the structure and function of flavoproteins. Riboflavin has been used in therapeutic approaches of various inborn errors of metabolism, notably in metabolic disorders resulting either from defects in proteins involved in riboflavin metabolism and transport or from defects in flavoenzymes. The scope of this review is to provide an updated perspective of clinical cases in which riboflavin was used as a potential therapeutic agent in disorders affecting mitochondrial energy metabolism. In particular, we discuss available mechanistic insights on the role of riboflavin as a pharmacological chaperone for the recovery of misfolded metabolic flavoenzymes.
In the past few decades, improved early diagnosis methods, technological developments and an increasing crosstalk between clinicians and researchers has led to the identification of an increasing number of inborn metabolic diseases. In these disorders, missense mutations are the most frequent type of genetic defects, frequently resulting in defective protein folding. A better understanding at the molecular level of protein misfolding and its role in disease has prompted the emergence of therapies based in the use of small molecules that have the ability to correct protein folding defects. Well-known cases are reported for phenylketonuria and Gaucher's disease. Most of these compounds have a specific mechanism of action interacting directly with a particular protein, the so called pharmacological chaperones. Among such small molecules are protein ligands, either natural substrates or synthetic derivatives, cofactors, competitive inhibitors, and agonist/antagonists. In this review we will start by briefly overviewing the mechanisms through which such ligands exert a stabilizing action, and then move on to an extended discussion on therapeutic approaches and use of vitamins and substrates to correct protein misfolding in metabolic disorders. Examples of vitamins that have been successfully prescribed to rescue some cases of inborn errors of metabolism will be presented. In particular, the role of riboflavin supplementation in the treatment of fatty acid β-oxidation disorders will be thoroughly analyzed, focusing on recent reports that shed light on the molecular basis of vitamin responsiveness. Moreover, we will highlight the latest studies that point to a synergistic effect of cofactors and metabolites in the rescue of defective fatty acid β-oxidation enzymes. The synergism of multiple small molecules may underlie a promising general pharmacological strategy for the treatment of metabolic diseases in general.
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