What is the significance of beta oxidation

The resulting enoyl CoAs undergo a sequential hydratase, dehydrogenase and thiolase reaction, leading to the formation of an NADH, an acetyl CoA and an acyl CoA which is two C-atoms shorter than the substrate of the cycle. We assume that MTP does not release the hydroxyacyl-CoA and ketoacyl-CoA intermediates in the matrix, but channels them from one active site to another. There is overlap between the substrates that can be converted by the two pathways.

However, MTP has a preference for longer chain lengths, while C4 and C6 substrates are uniquely converted by the crotonase branch. The reaction scheme in Figure 1 is translated into a set of 45 ODEs describing how the time derivatives of the concentrations of the variable metabolites depend on the enzyme rates. V MAT denotes the volume of the mitochondrial matrix. In equation 1 each reaction rate is specific for a certain combination of enzyme and substrate, since most enzymes catalyze multiple reactions.

The competing substrates inhibit each other's conversion competitively. This leads for instance to the following set of rate equation for crotonase: 2. All of the reactions are modeled as reversible, mostly of Michaelis-Menten type as above, and most parameters were taken from the literature and mainly based on biochemical characterization of purified rat-liver enzymes.

Some affinity and equilibrium constants were taken from rat-heart datasets. However, for enzymes of which only one isoenzyme exists, the affinity constants may be considered to be tissue-independent. Equilibrium constants do not depend at all on the enzyme that catalyzes the reaction. Three parameters, V cpt1 , V vlcad and V lcad , were estimated, since we could not find a reliable value in literature.

The model fulfills the criterion of microscopic reversibility, i. For instance, the overall equilibrium constant of the MTP reaction equals the product of the equilibrium constants in the crotonase branch. We note that most equilibrium constants are not far from 1; hence the pathway lacks a strong thermodynamic driving force and depends on the delivery of substrate upstream and the further conversion of products downstream. MTP dampens this thermodynamic hurdle by keeping the intermediate ketoacyl CoA bound.

Based on the above principles, we constructed a model of 45 variable metabolite concentrations, 56 reactions and parameters for detailed description see Text S1 , which predicts fluxes and metabolites in time and at steady state.

The predicted model outcome was validated with experiments on isolated rat-liver mitochondria. We measured the oxygen-consumption flux and the acyl-carnitine concentrations C4—C16 in time upon addition of palmitoyl CoA or palmitoyl carnitine.

Mitochondria were incubated with an excess amount of malate. Note that we did not include the carnitine moiety in the balance as it plays only a catalytic role. The same holds for the four carbons in citrate that are derived from malate. The uncertainty of the carbon balance leaves, however, room for other intermediates to contribute. We determined the time course of palmitoyl carnitine and used this as an input function for the substrate concentration in the model.

We assumed that the time course was similar for palmitoyl CoA when the latter was given as substrate see Text S1. Within the experimental error the experimental and modeled flux corresponded well on both substrates Figure 2A and Figure S1A. With palmitoyl carnitine, but not with palmitoyl CoA, a sharp initial overshoot was observed in the oxygen consumption flux, which was reproduced qualitatively by the model Figure S1D —G.

For the experiment with palmitoyl CoA as substrate, the dynamics of palmitoyl carnitine and the downstream intermediates myristoyl, octanoyl and hexanoyl carnitine C14, C8 and C6 were quantitatively reproduced by the model. For the experiment with palmitoyl carnitine as substrate, a qualitative correspondence was found, but the timescale and also concentrations differed Figure S1B—C and S2B.

We emphasize that the model parameters were not fitted to the experiments hence the correspondence between model and experiment is remarkably good. At time point zero the substrate palmitoyl CoA was added to uncoupled mitochondria in the presence of an excess amount of l -carnitine and malic acid. Samples for acyl carnitine analysis were taken at different time points.

Panel B: experimental acyl-carnitine concentrations in total samples, i. Panel C: acyl-carnitine concentrations simulated by the computer model, representing the weighted average over the matrix and extramitochondrial concentrations to allow direct comparison to the experiments. FA levels in plasma from obese people are often elevated [3] , [8]. We have simulated this by calculating the steady-state flux and metabolite concentrations as a function of the concentration of palmitoyl CoA, the substrate in the model Figure 3.

Panel D: Graphical representation of the differences between the computational models with and without competition. A priori either the accumulation of the CoA esters or the depletion of CoASH could cause the decline of flux in the standard model. To distinguish between these possibilities we fixed the CoASH concentration in the standard model and left the CoA esters free to accumulate Figure 3B , i. Accordingly, the drop in flux observed in the standard model is a result of CoASH depletion rather than of the high CoA-ester concentration.

At low concentrations of palmitoyl CoA the flux was lower than in the standard model, but it rose to the same maximum and the drop in flux occurred at a higher palmitoyl-CoA concentration Figure 3A and S3A.

Notably, we included the extensive competition in the system, as well as the qualitatively different pathways for conversion of enoyl CoAs. The parameters were based on biochemical analysis of individual enzymes and not fitted to obtain the desired metabolite and flux profiles. In this light, the correspondence between model predictions and experimental observations Figure 2 was remarkably good.

This allowed us to further explore the properties of the pathway. Hence, it could not be predicted from the properties of the individual enzymes, but resulted from the wiring of the entire network. It is tempting to speculate that this overload phenotype is at the basis of various diseases and may be one of the mechanisms of lipotoxicity.

In reality, surrounding pathways may protect the pathway from overload. In addition, the model revealed two possible protection mechanisms at the pathway boundaries. In the following we will discuss these protective mechanisms as well the possible role of fatty-acid overload in insulin resistance.

First, according to the model the flux decline could be prevented by decreasing the activity of CPT1. Accordingly, increased concentrations of the CPT1 inhibitor malonyl CoA as well as a decreased catalytic capacity of CPT1 have been observed experimentally [27] — [32]. The most convincing data are for malonyl CoA, which is increased in skeletal muscle of obese humans and rodents [27] , [28] , [31] , [32] as well as in liver tissue from obese mice [27].

Since the intermediate CoA esters are not detectable, it has been proposed that they are directly channeled from one active site to another [19] , [23] , [33] — [37]. The overall equilibrium constant of the lumped MTP reaction is 0. Besides the protection mechanisms found in the model, alternative mechanisms might be provided by surrounding pathways. Limited formation of palmitoyl CoA, either by inhibition of the synthetase reaction or due to a low cytosolic CoA concentration, would be very effective.

Little is known however, about regulation of palmitoyl CoA synthesis. Another option is upregulation of the pathways that consume the product acetyl CoA, such as the Krebs cycle or the formation of ketone bodies. Indeed, the liver can produce high amounts of ketone bodies when confronted with a high fat load [41]. So far, direct experimental evidence for the overload phenotype is lacking.

It is not unlikely, however, that it is at the basis of the well-known association of high FA levels to insulin resistance. In line with our model, it has been reported that this results in incomplete oxidation of fatty acids and accumulation of acyl carnitines and ketone bodies reviewed in [41]. It is unclear, however, if the CoA esters or the carnitine esters are responsible for this effect, or both. It is tempting to speculate that carnitine protects by trans-esterification of CoA esters to carnitine esters, which liberates CoASH and prevents accumulation of intermediates.

In vivo it is not clear, however, if carnitine prevents overload, since it plays a dual role: in the mitochondrial matrix it scavenges intermediate CoA esters, but in the cytosol it drives the uptake of acyl CoAs into the mitochondria. To further understand the interplay between glucose and FA metabolism and the quantitative role of various protective pathways, it will be of key interest to link the new model with partially existing models of glucose metabolism, TCA cycle, respiration, ketone-body synthesis and FA synthesis [43] — [46].

This should be the next step in elucidating the mechanisms behind acquired and inborn diseases of glucose and FA metabolism. The computational model was built and analyzed in Mathematica Wolfram. Time simulations were done with the algorithm NDSolve. Steady states were calculated by setting all time derivatives to zero and solving the resulting set of non-linear equations with the algorithm FindRoot.

FindRoot is a root-finding algorithm combining damped Newton's method, the secant method and Brent's method. We have no indications for alternative steady-state solutions, since different initial conditions led to identical steady states.

As an input for the steady-state root-finding algorithm we used the endpoint of a time simulation. Your browser does not have JavaScript enabled and some parts of this website will not work without it. For the best experience on the Abcam website please upgrade to a modern browser such as Google Chrome.

Fatty acids provide highly efficient energy storage, delivering more energy per gram than carbohydrates like glucose. Fatty acids are activated for degradation by conjugation with coenzyme A CoA in the cytosol. The long-chain fatty-acyl-CoA is then modified by carnitine palmitoyltransferase 1 CPT1 to acylcarnitine and transported across the inner mitochondrial membrane by carnitine translocase CAT.

Sets of 4 enzymes an acyl dehydrogenase, a hydratase, a hydroxyacyl dehydrogenase, and a lyase specific for different chain lengths very long chain, long chain, medium chain, and short chain are required to catabolize fatty acids completely.

Inheritance for all fatty acid oxidation defects is autosomal recessive Autosomal Recessive Genetic disorders determined by a single gene Mendelian disorders are easiest to analyze and the most well understood. If expression of a trait requires only one copy of a gene one allele This deficiency is the most common defect in the beta-oxidation cycle and has been incorporated into expanded neonatal screening Screening Tests for Newborns Screening recommendations for newborns vary by clinical context and state requirements.

Blood typing is indicated when the mother has type O or Rh-negative blood or when minor blood antigens Clinical manifestations typically begin after 2 to 3 months of age and usually follow fasting as little as 12 hours.

Patients have vomiting and lethargy that may progress rapidly to seizures, coma, and sometimes death which can also appear as sudden infant death syndrome. During attacks, patients have hypoglycemia, hyperammonemia, and unexpectedly low urinary and serum ketones. Metabolic acidosis is often present but may be a late manifestation. Diagnosis of MCADD is by detecting medium-chain fatty acid conjugates of carnitine in plasma or glycine in urine or by detecting enzyme deficiency in cultured fibroblasts; however, DNA testing can confirm most cases.

Symptoms and signs include thirst, lethargy, dry mucosa, decreased urine output, and, as the degree Prevention is a low-fat, high-carbohydrate diet and avoidance of prolonged fasting.

Cornstarch therapy is often used to provide a margin of safety during overnight fasting. This deficiency is the 2nd most common fatty acid oxidation defect. It shares many features of MCADD, but patients may also have cardiomyopathy; rhabdomyolysis, massive creatine kinase elevations, and myoglobinuria with muscle exertion; peripheral neuropathy; and abnormal liver function.