Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 432

Biochemistry

Spring 2009

Lecture Notes: 2 February

© R. Paselk 2006
 
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Fatty Acid Biosynthesis, cont.

PYRUVATE-MALATE CYCLE

The reactions of fatty acid synthesis all take place in the cytosol, but acetyl-CoA is made in the mitochondria and can't cross the inner membrane. The Pyruvate-Malate Cycle (Citrate-Pyruvate Cycle) [overhead] (packet) is used to take acetyl- groups to the cytosol while simultaneously providing a source of NADPH from NADH, and thus coupling fatty acid synthesis to Glycolysis. Note that the acetyl-CoA is first joined to oxaloacetate to make citrate which is readily transported out of the mitochondria using a co-transporter. The citrate is then cleaved to acetyl-CoA and oxaloacetate, a process requiring ATP to make it favorable (recall the condensation was spontaneous). Acetyl-CoA for fatty acid synthesis is now available in the cytosol, but oxaloacetate must be regenerated for the mitosol.

The cytosolic oxaloacetate is now dehydrogenated to give malate and NAD+. Malate is next oxidized by Malic enzyme to give pyruvate in a reaction which also provides NADPH for use in biosynthesis. (Note that NADH generated in Glycolysis is "converted" to NADPH for F.A. synthesis in these two reactions, while simultaneously regenerating the NAD+ needed to continue Glycolysis!) The pyruvate can now cross into the mitosol to be used in regenerating oxaloacetate.

Now we have all of the pieces of fatty acid biosynthesis starting from glucose. Let's look at the integration of the various pathways involved: Glycolysis, hexose monophosphate shunt, Pyruvate-malate shuttle, and Fatty acid biosynthesis. Notice the provision of reducing equivalents, redox balance and provision of required cytosolic ATP's as well as carbon source.

Fatty Acid Modification

ELONGATION OF FATTY ACIDS

If the Fatty Acid Synthetase Complex only makes palmitate where do the rest of the fatty acids come from? Of course palmitate can be shortened by -oxidation. For longer fatty acids there is a fatty acid elongation system localized on the ER. The same reactions occur as in the Synthetase, but now have individual enzymes. Palmitate is first activated to palmitoyl-CoA. The enzymes prefer C-16 or less as substrate; thus the major product is stearoyl-CoA. However longer unsaturated fatty acids will also bind (the kinking of the cis double bond makes them effectively shorter), so unsaturated fatty acids of 20, 22, and 24-C's are also made. Thus most longer fatty acids are polyunsaturated.
A second system for fatty acid elongation exists in the mitosol, probably for provision of long fatty acids for mitochondrial structure. This system uses most of the same activities of -oxidation, but an NADPH dependent Enoyl-CoA reductase replaces the FAD dependent dehydrogenase.

FATTY ACID DESATURATION

Plants and animals differ in where double bonds are introduced into fatty acids.

Plants put in 9 (a 9-10 double bond) and then can put in double bonds at three carbon intervals towards the tail (12, 15). They can also add 6, but not common.

Animals also start with 9, then can add at three carbon intervals toward carboxy end (6 and 3). Animals cannot add towards the tail. Therefore animals cannot make linoleoyl-CoA (9, 12; C18) but can make oleoyl-CoA (9; C18). Animals must therefore take in plant products (either directly as herbivores, or indirectly by eating herbivores) to acquire essential unsaturated fatty acids such as linoleic and arachadonic acids.

Both plants and animals use mixed function oxidases (simultaneously oxidize two substrates): Acyl-CoA desaturases localized on the ER. Similar mixed function oxidases are also used to modify structural components of cells, hormones etc. We will use the acyl-CoA desaturase as an example for this group of enzymes. In the acyl-CoA desaturase reaction molecular oxygen is used to oxidize both a fatty acid and NADH, each providing two of the the four electrons needed by the oxygen:

structural diagram of the acyl-CoA desaturase reaction
The mammalian acyl desaturases are components in mini-electron transport systems on the surface of the endoplasmic reticulum, for example the 9-fatty acyl-CoA desaturase complex:

diagram of the electron transport system for acyl-CoA desaturase

 


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Last modified 31 January 2009