Biochemical Competition Makes Fatty-Acid β-Oxidation Vulnerable to Substrate Overload

Karen Van Eunen,Catharina M L Touw,Sander M Houten,Sereh M J Simons,Bert K Groen,Dirk-Jan Reijngoud,Aycha Bleeker,Barbara M Bakker,Gijs Den Besten,Albert Gerding,Klaas Krab

doi:10.1371/journal.pcbi.1003186

Abstract

Fatty-acid metabolism plays a key role in acquired and inborn metabolic diseases. To obtain insight into the network dynamics of fatty-acid β-oxidation, we constructed a detailed computational model of the pathway and subjected it to a fat overload condition. The model contains reversible and saturable enzyme-kinetic equations and experimentally determined parameters for rat-liver enzymes. It was validated by adding palmitoyl CoA or palmitoyl carnitine to isolated rat-liver mitochondria: without refitting of measured parameters, the model correctly predicted the β-oxidation flux as well as the time profiles of most acyl-carnitine concentrations. Subsequently, we simulated the condition of obesity by increasing the palmitoyl-CoA concentration. At a high concentration of palmitoyl CoA the β-oxidation became overloaded: the flux dropped and metabolites accumulated. This behavior originated from the competition between acyl CoAs of different chain lengths for a set of acyl-CoA dehydrogenases with overlapping substrate specificity. This effectively induced competitive feedforward inhibition and thereby led to accumulation of CoA-ester intermediates and depletion of free CoA (CoASH). The mitochondrial [NAD+]/[NADH] ratio modulated the sensitivity to substrate overload, revealing a tight interplay between regulation of β-oxidation and mitochondrial respiration.

Highlights

Pathophysiological mechanisms underlying acquired and inborn metabolic diseases, such as type-2 diabetes and deficiencies in the fatty-acid oxidation, are largely elusive
Model construction We constructed a model for mitochondrial FA b-oxidation with all enzyme reactions and transporters (Figure 1)
We added the long-chain acyl-CoA dehydrogenase (LCAD), which is involved in rodent boxidation [15,16,17,18]

Summary

Introduction

Pathophysiological mechanisms underlying acquired and inborn metabolic diseases, such as type-2 diabetes and deficiencies in the fatty-acid oxidation, are largely elusive. Fatty-acid (FA) b-oxidation is a prime example of a pathway involved in many diseases, but for which it is difficult to acquire a complete and quantitative view on the relation between metabolite concentrations and fluxes. It has been argued that an imbalance between cellular FA uptake and oxidation leads to accumulation of FAs and other lipid molecules in the cytosol, which in turn causes insulin resistance [4,5]. Others showed that a functioning acyl-CoA uptake into mitochondria is needed to develop insulin resistance, leading to the hypothesis that intermediates of FA b-oxidation are part of the problem [6]. A more direct view on the dynamics of b-oxidation intermediates is urgently needed

Methods

Results

Conclusion