The Metabolism of Lipids
In this project I will discuss the anabolism
and catabolism of lipids and steroids. The steps in the pathways
from reactants to products will be mentioned. Remember that anabolism
is the synthesis of organic molecules and catabolism is the use of these
molecules.
Catabolic Pathways and Lipids
Energy requirements for biological organisms
are extensive. Average recommended caloric allowances for adults
range from 2500 to 3500 kilo calories per day, based on body mass, age,
and activity levels. Most organisms are heterotrophs, and rely on
plant and animal sources for their energy supply. One source of energy
is lipids, provided by numerous food sources, particularly dairy products.
The following pathway reduces lipids to the end product Acetyl-CoA.
Catalysis of Lipids to Acetyl-CoA and Reduced
Electron Carriers
Acetyl-CoA comes from the Beta-oxidation
of lipids. It is called beta-oxidation because the beta-carbon is
always oxidized as the first step in the process. The process involves
a series of steps that each releases a two-carbon fragment, which is acetyl-CoA.
This beta-oxidation pathway is cyclic -- in each step, the substrate is
an acyl-CoA, which is shortened (by formation of the acetyl-CoA) by two
carbons. The product, a new, shorter acyl-CoA, then undergoes
the same reaction again.
From chapter 18 in the text, there are
four steps in each time a new acetyl-CoA is formed from an acyl-CoA substrate:
-
1: Dehydrogenation of the saturated fatty
acyl-CoA to give an enoyl derivative, then:
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2: Hydration of the resulting double bond
and hydroxylation of the beta-carbon;
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3: Dehydrogenation of the hydroxyl group;
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4: A second molecule of acetyl-CoA attacks
the beta-carbon, releasing acetyl-CoA and a fatty acyl-CoA (with the number
of carbons
dropped by two).
Anabolic Pathways to Lipids
Some microorganisms (autotrophs) produce
their own food, and they can use the following pathways to create lipids.
These pathways are also responsible for the creation of steroids, which
are a physiological requirement for most organisms. The
synthesis of fatty acids is not simply the reverse of the catabolism pathways.
Different electron carriers are used, and there are differences in the
stereochemistry of the reactions. A main similarity is that two
carbon fragments are added to form the new lipids. Since this
topic has not been studied in significant detail, we will, for the time
being, study the process by which acetyl-CoA is used to synthesize lipids.
Referring again to chapter 18, we can discuss
the process of lipid synthesis from acetyl-CoA.
To begin, palmitate is synthesized from
acetyl-CoA.
Palmitate is elongated, two carbons at
a time.
The resulting lipid is desaturated.
The first step, synthesis of palmitate,
occurs as a cycle of four reactions:
-
1: Condensation
-
2: Reduction
-
3: Dehydration
-
4: Reduction
This cycle repeats four times. Elongation
of the chain occurs by a condensation, then a reduction, a dehydration,
and another reduction. This results in a saturated fatty acyl-CoA
two carbons longer than the original substrate. The desaturation
occurs by enzyme systems such as fatty acyl-CoA desaturase.
Research Involving Metabolism and Lipids
Among the leading causes of health
problems among neonates (infants under 30 days of age) are metabolic
disorders. These include problems associated with breakdown and storage
of lipids. Research is under way that would help resolve problems
with lipid metabolism, such as with enzyme replacement therapy.
Project Links
An
overview of metabolism -- Stan's page
Oxidative
Phosphorylation...and much more -- Leah's page
Carbohydrate
Catabolism -- Rod's Page
Metabolism
-- Project 7 Main Page
