Metabolic inflexibility: when mitochondrial indecision leads to metabolic gridlock

Abstract

Normal energy metabolism is characterized by periodic shifts in glucose and fat oxidation, as the mitochondrial machinery responsible for carbon combustion switches freely between alternative fuels according to physiological and nutritional circumstances. These transitions in fuel choice are orchestrated by an intricate network of metabolic and cell signaling events that enable exquisite crosstalk and cooperation between competing substrates to maintain energy and glucose homeostasis. By contrast, obesity-related cardiometabolic diseases are increasingly recognized as disorders of metabolic inflexibility, in which nutrient overload and heightened substrate competition result in mitochondrial indecision, impaired fuel switching, and energy dysregulation. This Perspective offers a speculative view on the molecular origins and pathophysiological consequences of metabolic inflexibility.

Figure 1. Nutrient Sensing and Signaling Regulate Substrate Selection during Fasting and Feeding

Glucose and fatty acids serve as the primary catabolic substrates that provide acetyl-CoA to the tricarboxylic acid cycle. The pathways of glucose and fat oxidation are reciprocally regulated by several key metabolic intermediates and signals. During fasting, elevated acetyl-CoA derived from high rates of β-oxidation lowers glucose oxidation by allosterically inhibiting PDH and by activating its inhibitory kinase, PDK. Conversely, feeding and glucose surplus restrict fat oxidation by increasing production of malonyl-CoA, which inhibits CPT1. Citrate acts as a signal of plenty that limits glycolytic flux by inhibiting PFK and lowers β-oxidation by giving rise to cytoplasmic acetyl-CoA and malonyl-CoA via CL and ACC, respectively. During periods of energy deficit, an increase in the cellular AMP/ATP ratio activates AMPK, which phosphorylates and inhibits ACC while also activating MCD, thereby relieving malonyl-CoA-mediated inhibition of CPT-1 and promoting fat oxidation. Catabolism of branched-chain amino acids (BCAA) is regulated by BCKD, which is feedback inhibited by acyl-CoA products of the complex due to activation of its inhibitory kinase, BCK. Increased cellular concentrations of pyruvate, BCAA, and fatty acyl-CoAs promote their own catabolism by antagonizing the inhibitory actions of PDK, BCK, and malonyl-CoA, respectively. ACC, acetyl-CoA carboxylase; ACS, acyl-CoA synthetase; AMPK, 5’ AMP-activated kinase; BCAA, branched-chain amino acids; BCKD, branched-chain ketoacid dehydrogenase; BCK, BCKD kinase; CL, citrate lyase; CPT1, carnitine palmitoyltransferase 1; ETC, electron transport chain; G6P, glucose 6 phosphate; HK, hexokinase; LCAC, long-chain acylcarnitine; LCACoA, long-chain acyl-CoA; MCD, malonyl-CoA decarboxylase; PFK, phosphofructokinase; PDH, pyruvate dehydrogenase; PDK, PDH kinase. Red indicates inhibition; green indicates activation; circles are transporters.

Figure 2. Mitochondrial Indecision Results in Metabolic Inflexibility

In healthy, metabolically flexible states, consumption of a high-carbohydrate (CHO) meal together with a small rise in blood insulin levels elicit a surge in the respiratory quotient (RQ; VCO₂/VO₂), indicative of a robust shift from fatty acid to glucose oxidation. During the postprandial (PP) hours following a meal, mitochondria consume a mixture of fats and carbohydrate. Progression toward the postabsorptive (PA) state and prolonged fasting are accompanied by increased fat oxidation and a corresponding decline in the RQ. Mitochondrial capacity to switch freely between oxidative fuels depending on the nutritional context is lost in obese, metabolically inflexible individuals. Persistent oxidation of a mixture of carbon fuels increases mitochondrial congestion and risk of metabolic hazard.

Figure 3. Nutrient Overload Leads to Mitochondrial Gridlock and Cellular Dysfunction

Glucose, fatty acids, and branched-chain amino acids are degraded to acyl-CoA intermediates that fuel the mitochondrial tricarboxylic acid cycle (TCAC). All catabolic roadways in the mitochondria, including the TCAC, lead to the production of reducing equivalents (NADH) that feed the electron transport chain (ETC), thereby permitting ATP regeneration to support cellular work and energy expenditure. In healthy states, carbon traffic at the crossroads marked by PDH, CPT1, and BCKD is coordinately and reciprocally regulated by an intricate network of metabolic signals that match energy supply to ATP demand. High rates of fat oxidation inhibit glucose and BCAA catabolism and vice versa, thereby preventing mitochondrial congestion when ATP consumption is low. Chronic overnutrition causes metabolic confusion and signal failure, resulting in unabated influx of surplus fuel and an ensuing traffic jam at several critical bottlenecks where the roadways converge. As mitochondrial traffic reaches a state of grid-lock, membrane potential rises and accumulating electrons and acyl-CoAs are diverted toward ROS generation and PTMs such as glutathionylation and lysine acetylation, which further disrupts nutrient sensing and signaling. If these road hazards are not sufficiently managed by the mitochondrial buffering and repair systems, mounting irreversible damage to cellular macromolecules leads to organ dysfunction.