Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438	Introductory Biochemistry	Spring 2007
Lecture Notes: 18 April	© 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.

FYI - 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 6). 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. so 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: The mammalian acyl desaturases are components in mini-electron transport systems on the surface of the endoplasmic reticulum, for example the D9-fatty acyl-CoA desaturase complex:

REGULATION OF FATTY ACID METABOLISM

Fatty acid metabolism is regulated both hormonally and via feed-back inhibition and feed-forward activation. Thus mobilization of free fatty acids from the adipose tissue results from low insulin levels. The free fatty acids are then transported through the blood to the rest of the body including the liver. In the liver fatty acid oxidation and ketone body synthesis is activated by glucagon. Note that glucagon and insulin levels are opposite: high insulin =low glucagon and vice-versa. So for low insulin will also have high glucagon, thus fatty acids will be released from the adipose and will be converted in the liver into ketone bodies.

The regulation of fatty acid oxidation, fatty acid synthesis and ketone body synthesis in the liver is summarized in the figure:

Note that a LACK of insulin results in a release of fatty acids from adipose.

Nitrogen Metabolism

Protein catabolism and anabolism are often out of sync - either no additional protein is needed or the amino acid composition of the synthesized proteins is not identical to the protein being hydrolyzed. Neither protein nor amino acids are stored as such. Thus organisms must frequently degrade excess amino acids. Two paths may be available: deaminate the unneeded amino acids and breakdown the carbon skeletons for energy or for storage as fat or carbohydrate and eliminate the nitrogen; or the nitrogen may be transferred to another carbon backbone to make a needed amino acid.

Nitrogen can be eliminated in a variety of forms, depending on the physiological conditions experienced by the organism:

• Ammonia and/or ammonium ion. This occurs to some extent in all organisms. However ammonia is quite toxic so it must be kept at low concentrations in the body. Thus ammonia is used as the major excretory product in organisms with an essentially unlimited amount of water available to dilute it: plants, fish, many amphibians.
• Urea. Animals with readily available water (for drinking) use urea as their major nitrogenous waste product. Urea is non-toxic at low (<1 M) concentrations and is extremely soluble (>10 M). It is thus an excellent waste in liquid form. Used by mammals. (Lungfish have a variable environment, water to mud, and switch back and forth between ammonia and urea as appropriate!)
• Uric acid. Animals living in deserts, environmental or physiological, use uric acid. Uric acid is quite insoluble, therefore water is not needed to excrete it. Unfortunately insolubility also means it has a tendency to crystallize out in joints etc. (e.g. small amounts in humans can lead to gout). Uric acid is thus used by reptiles and birds, including species living in or near water. The problem here is these animals all start life in a "physiological desert" within their eggs. Many birds of course also have the "migration problem" - they must fly immense distances and cannot afford to carry excess water (weight). (Spiders use an even more insoluble compound, xanthine.)

AMINO ACID TRANSAMINATION & DEAMINATION

Amino Acid Deamination: For any of these products nitrogen must first be removed from the amino acids. Ammonia is often an intermediate if not the final product in all of these organisms. Most ammonia results from the deamination of a single amino acid: glutamate, via the Glutamate dehydrogenase reaction:

This enzyme is somewhat unusual in that it can use either NADH or NADPH. Glutamate DH is activated by ADP and inhibited by GTP in vitro so they may regulate it in vivo. Of course these nucleotides would not provide a highly sensitive regulation, activity would only loosely follow energy charge.

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Last modified 18 April 2007