Palmitoyl-CoA is an
acyl-CoA thioester. It is an "activated" form of
palmitic acid
Palmitic acid (hexadecanoic acid in IUPAC nomenclature) is a fatty acid with a 16-carbon chain. It is the most common saturated fatty acid found in animals, plants and microorganisms.Gunstone, F. D., John L. Harwood, and Albert J. Dijkstra. The L ...
and can be transported into the mitochondrial
matrix by the
carnitine shuttle system (which transports fatty
acyl-CoA molecules into the
mitochondria), and once inside can participate in
beta-oxidation. Alternatively, palmitoyl-CoA is used as a substrate in the biosynthesis of
sphingosine
Sphingosine (2-amino-4-trans-octadecene-1,3-diol) is an 18-carbon amino alcohol with an unsaturated hydrocarbon chain, which forms a primary part of sphingolipids, a class of cell membrane lipids that include sphingomyelin, an important phos ...
(this biosynthetic pathway does not require transfer into the mitochondria).
Biosynthesis
Palmitoyl CoA formed from palmitic acid, in the reaction below.
This reaction is often referred to as the "activation" of a fatty acid. The activation is catalyzed by
palmitoyl-coenzyme A synthetase and the reaction proceeds through a two step mechanism, in which palmitoyl-AMP is an intermediate. The reaction is driven to completion by the exergonic hydrolysis of pyrophosphate.
The activation of fatty acids occurs in the cytosol and beta-oxidation occurs in the mitochondria. However, long chain fatty acyl-CoA cannot cross the mitochondrial membrane. If palmitoyl-CoA is to enter the mitochondria, it must react with carnitine in order to be transported across:
Palmitoyl-CoA + Carnitine <-> Palmitoyl-Carnitine + CoA-SH
This transesterification reaction is catalyzed by
carnitine palmitoyl transferase. Palmitoyl-Carnitine may translocate across the membrane, and once on matrix side, the reaction proceeds in reverse as CoA-SH is recombined with palmitoyl-CoA, and released. Unattached carnitine is then shuttled back to the cytosolic side of mitochondrial membrane.
Beta-Oxidation
Once inside the mitochondrial matrix, palmitoyl-CoA may undergo
β-oxidation
In biochemistry and metabolism, beta-oxidation is the catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA, which enters the citric acid cycl ...
. The full oxidation of palmitic acid (or palmitoyl-CoA) results in 8 acetyl-CoA's, 7
NADH
Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an aden ...
, 7 , and 7
FADH2. The full reaction is below:
Palmitoyl-CoA +7CoA-SH + 7NAD+ + 7FAD -> 8Acetyl-CoA + 7NADH + 7H+ + 7FADH2
Sphingolipid Biosynthesis
Palmitoyl-CoA is also the starting substrate, along with Serine, for sphingolipid biosynthesis. Palmitoyl CoA and Serine participate in a condensation reaction catalyzed by
serine C-palmitoyltransferase (SPT), in which
3-ketosphinganine is formed. These reactions occur in the cytosol.
Additional images
Image:Sphk1 diag b.jpg, Synthesis
Image:Palmitic acid.png, Palmitic acid
Palmitic acid (hexadecanoic acid in IUPAC nomenclature) is a fatty acid with a 16-carbon chain. It is the most common saturated fatty acid found in animals, plants and microorganisms.Gunstone, F. D., John L. Harwood, and Albert J. Dijkstra. The L ...
Image:Coenzym A.svg, Coenzyme A
See also
Coenzyme A CoA
References
Thioesters of coenzyme A
Organophosphates
Palmitate esters
{{ester-stub