Biochemical competition makes fatty-acid β-oxidation vulnerable to substrate overload.

Fatty acid β-oxidation is a central metabolic process, yet its vulnerability to substrate overload remains poorly understood. Prior research has shown that disruptions in this pathway contribute to both inherited and acquired metabolic disorders. However, the precise mechanisms by which substrate overload affects β-oxidation flux have not been fully resolved. Existing models lack the dynamic and kinetic detail needed to capture how enzyme saturation influences pathway behavior. This gap motivated the development of a computational model to simulate β-oxidation under fat overload conditions. The model integrates enzyme kinetics and substrate competition to explore how these factors influence flux and intermediate accumulation. No prior work had resolved how acyl-CoA chain-length competition impacts β-oxidation efficiency. This study aims to address these uncertainties by analyzing the interplay between enzyme specificity and substrate availability.

The study aims to explore how fatty-acid β-oxidation responds to substrate overload using a detailed computational model. The researchers sought to understand whether enzyme saturation and substrate competition could lead to metabolic bottlenecks. They focused on the role of acyl-CoA dehydrogenases in processing fatty acids of different chain lengths. The model was tested against experimental data from isolated rat liver mitochondria to ensure accuracy. The goal was to simulate the effects of increased palmitoyl-CoA concentrations to mimic obesity-related fat overload. The researchers also aimed to determine how NAD⁺/NADH ratios influence the sensitivity of β-oxidation to substrate overload. This approach allowed them to explore the interplay between β-oxidation and mitochondrial respiration. The study's findings may help explain why fatty acid metabolism becomes impaired under conditions of high substrate availability.

The researchers constructed a computational model of fatty-acid β-oxidation using enzyme-kinetic equations and experimentally derived parameters. The model included reversible and saturable enzyme reactions specific to rat liver mitochondria. They validated the model by comparing predicted β-oxidation flux and acyl-carnitine concentrations to experimental data from isolated mitochondria. The model was then used to simulate fat overload by increasing palmitoyl-CoA concentrations. The researchers analyzed how different chain-length acyl-CoAs competed for dehydrogenase enzymes. They examined the resulting changes in flux, intermediate accumulation, and free CoA depletion. The model also incorporated the effects of the mitochondrial [NAD⁺]/[NADH] ratio on pathway sensitivity. These simulations provided insights into the regulatory dynamics of β-oxidation under overload conditions.

The model accurately predicted β-oxidation flux and acyl-carnitine time profiles when compared to experimental data from isolated rat liver mitochondria. When palmitoyl-CoA concentrations were increased to simulate obesity, β-oxidation flux decreased significantly. This drop in flux was accompanied by the accumulation of CoA-ester intermediates and a depletion of free CoA. The primary cause was competitive feedforward inhibition among acyl-CoAs of different chain lengths. This competition limited the availability of dehydrogenase enzymes, reducing overall pathway efficiency. The mitochondrial [NAD⁺]/[NADH] ratio played a key role in modulating the sensitivity to substrate overload. Higher NADH levels increased the pathway's vulnerability to overload conditions. These findings suggest that enzyme saturation and substrate competition are critical in determining β-oxidation stability.

The study's findings suggest that fatty-acid β-oxidation is vulnerable to substrate overload due to competitive enzyme saturation. The model demonstrated that increasing palmitoyl-CoA concentrations leads to flux reduction and intermediate accumulation. This behavior arises from the competition between acyl-CoAs of different chain lengths for shared dehydrogenase enzymes. The researchers propose that this competition effectively induces feedforward inhibition, reducing pathway efficiency. Free CoA depletion and CoA-ester accumulation were observed as direct consequences of this overload. The mitochondrial [NAD⁺]/[NADH] ratio modulates the sensitivity of β-oxidation to overload conditions. These results highlight the tight interplay between β-oxidation and mitochondrial respiration. The authors suggest that substrate competition and enzyme saturation are key factors in determining pathway stability under high-fat conditions.