The intracellular concentration of free unbound acyl-CoA esters is tightly controlled by feedback inhibition of the acyl-CoA synthetase and is buffered by specific acyl-CoA binding proteins. Excessive increases in the concentration are expected to be prevented by conversion into acylcarnitines or by hydrolysis by acyl-CoA hydrolases. Under normal physiological conditions the free cytosolic concentration of acyl-CoA esters will be in the low nanomolar range, and it is unlikely to exceed 200 nM under the most extreme conditions. The fact that acetyl-CoA carboxylase is active during fatty acid synthesis (Ki for acyl-CoA is 5 nM) indicates strongly that the free cytosolic acyl-CoA concentration is below 5 nM under these conditions. Only a limited number of the reported experiments on the effects of acyl-CoA on cellular functions and enzymes have been carried out at low physiological concentrations in the presence of the appropriate acyl-CoA-buffering binding proteins. Re-evaluation of many of the reported effects is therefore urgently required. However, the observations that the ryanodine-senstitive Ca2+-release channel is regulated by long-chain acyl-CoA esters in the presence of a molar excess of acyl-CoA binding protein and that acetyl-CoA carboxylase, the AMP kinase kinase and the Escherichia coli transcription factor FadR are affected by low nanomolar concentrations of acyl-CoA indicate that long-chain acyl-CoA esters can act as regulatory molecules in vivo. This view is further supported by the observation that fatty acids do not repress expression of acetyl-CoA carboxylase or Delta9-desaturase in yeast deficient in acyl-CoA synthetase.

Role of long-chain fatty acyl-CoA esters in the regulation of metabolism and in cell signalling.

Structural and functional features of different types of cytoplasmic fatty acid-binding proteins

Control of mitochondrial beta-oxidation flux.

A series of noncovalent complexes formed between the 86 residue acyl CoA binding protein (ACBP) and a series of acyl CoA derivatives has been studied by electrospray ionization mass spectrometry. Conditions were found under which CoA ligands can be observed in the mass spectrometer bound to ACBP. Despite the very low dissociation constants (10-7 to 10-10 M) of the acyl CoA ligand complexes high ratios of ligand-to-protein concentration in the electrospray solution were found to increase the proportion of intact complex observed in the spectrum. Variation in the length of the hydrophobic acyl chain of the ligand (C16, C12, C8, C0) resulted in similar proportions of complex observed in the mass spectrum even though significant variation in solution dissociation constants has been measured. A substantially reduced proportion of complex was, however, found for the mutant proteins, Y28N, Y31N, and Y73F, lacking tyrosine residues involved in critical interactions with the CoA ligand. These results have been int...

Probing the Nature of Noncovalent Interactions by Mass Spectrometry. A Study of Protein−CoA Ligand Binding and Assembly

Lipid metabolism in the lactating mammary gland.

Bovine liver was shown to contain a hitherto undescribed medium-chain acyl-CoA-binding protein. The protein co-purifies with fatty-acid-binding proteins, but was, unlike these proteins, unable to bind fatty acids. The protein induced synthesis of medium-chain acyl-CoA esters on incubation with goat mammary-gland fatty acid synthetase. The possible function of the protein is discussed.

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H Schulenberg

Papers

A novel acyl-CoA-binding protein from bovine liver. Effect on fatty acid synthesis.