3-hydroxybutyrate proton production

One of the steps in fat metabolism is the hydration of crotonate to yield 3-hydroxybutyrate. The reaction occurs by addition of —OH to the Si face at C3, followed by protonation at C2, also from the Si face. Draw the product of the reaction, showing the stereochemistry of each step. 

In individuals with a deficiency in methylacetoacetyl CoA thiolase (MACT), the conversion of 2-methylacetoacetyl-CoA into acetyl-CoA and propionyl-CoA is inhibited and an accumulation of abnormal catabolic products is observed in the urine. Williams etal. have successfully used both ID and 2D H NMR spectroscopy to investigate the urinary metabolites of two unrelated patients with this disorder. The urine spectra from both patients clearly showed the presence of both 2-methyl-3-hydroxybutyrate and tiglylglycine, which is characteristic for MACT deficiency due to the build-up of metabolites close to the position of enzyme deficiency. However, at 360 MHz, the H NMR spectra are quite crowded and difficult to interpret fully. This difficulty was overcome by the use of the 2D JRES H NMR experiment, which resolved all the proton-proton couplings into the second dimension, allowing much clearer spectral interpretation. In addition to the spin-echo and 2D H J-resolved NMR spectra, 2D H COSY NMR spectroscopy was used to determine the spin-spin coupling connectivities between the protons in the various urinary components. 

According to Kambe et al., the mechanism of reaction consists not in the reduction of methyl acetoacetate by photochemically produced hydrogen, but in the electrochemical reduction of the C=0 bond and proton by photogenerated electrons. Thus, the hydrogen production sites on the surface of Raney Ni are different from methyl 3-hydroxybutyrate production sites. 

As is well known, the dietary carbohydrates are normally catabolized to give the neutral end products carbon dioxide and water. The intermediately formed organic acids such as lactic acid or tri- and dicarbonic acids of the Krebs cycle solely influence the acid-base balance if they are excreted in their ionic form, leaving behind the protons that would normally be oxidized together with the acidic anion to the neutral end products mentioned above. The same is true for the fatty acids, originating from the dietary fats. Under special circumstances, the fatty acid, degradation leads to the accumulation of ketone bodies. The excretion of acetoacetic and of jS-hydroxybutyric acid in their ionic form results in acidosis, as observed in diabetes. Under normal conditions, however, there is no influence of dietary fat on the acid-base balance. 

II) Similarly, protons are formed during the production of acetoacetate and 3-hydroxybutyrate from fatty acids (Chapter 33)... 

Maximum concentrations of the species were obtained when the reaction was carried out at low pressure with removal of the water formed in the course of the reaction. In this way the Cr species stabilized by ethylene glycol was stable at room temperature for several days. When the reaction was studied in dioxan, a variety of hydroxy-carboxylic acids were shown to give rise to Cr signals whereas oxalic acid and mercapto-carboxylic acids do not. The kinetics of the oxidation of a-hydroxybutyric and other a-hydroxy-acids in water have been investigated,with no evidence for intermediate complex formation. Failure to detect any reduction of HgCl2 has been attributed to the absence of free radicals, the authors considering the rate-determining process to involve the formation of a carbonium ion which yields the ketone product by release of a proton. 

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