Ch 24: Lipid Biosynthesis
ceramide
A precursor for all sphingolipids
thioesterase (TE)
Enzyme that releases palmitate in the final reaction of fatty acid synthesis
Technically yes, but only if it stays in the box. Once it is eaten, it will be converted to fat.
Excess carbohydrates (via pyruvate) can provide a significant source of acetyl-CoA for storage as fat. Thus, are food labeled "fat free" actually fat free?
Synthesis of glycerophospholipids in begins with the formation of phosphatidic acid, which may be formed from either dihydroxyacetone phosphate or glycerol as shown.
What are the main precursors for the formation of glycerophospholipids?
diacylglycerol, ethanolamine, and CTP
What are the precursors of phosphatidyl-ethanolamine?
diacylglyercol, choline, CTP
What are the precursors of phosphatidylcholine?
Longer fatty acid chains are made through elongation reactions, which occur both in the mitochondria and at the surface of the endoplasmic reticulum (ER). The ER reactions are actually quite similar to those we have just discussed: addition of two-carbon units at the carboxyl end of the chain by means of oxidative decarboxylations involving malonyl-CoA. As was the case for the fatty acid synthase, this decarboxylation provides the thermodynamic driving force for the condensation reaction. The mitochondrial reactions involve addition (and subsequent reduction) of acetyl units. These reactions are essentially a reversal of fatty acid oxidation, with the exception that NADPH is utilized in the saturation of the double bond, instead of FADH2.
What happens in fatty acid elongation reactions?
The carboxylation of acetyl-CoA to form malonyl-CoA is essentially irreversible and is the committed step in the synthesis of fatty acids. The reaction is catalyzed by acetyl-CoA carboxylase, which contains a biotin prosthetic group. This carboxylase is the only enzyme of fatty acid synthesis in animals that is not part of the multienzyme complex called fatty acid synthase. Overall, the reaction uses a molecule of acetyl-CoA, ATP and HCO3- as source of carbon dioxide/
What is the committed step for the synthesis of fatty acids?
8 acetyl-CoA + 14 NADPH + 7 ATP >>>>> palmitate + 14 NADP+ + 8 CoA + 6 H2O + 7 ADP + 7 Pi.
What is the overall reaction for the synthesis of palmitate?
active state is unphosphorylated. When there is a need for ATP, it will be more phosphorylated and less active. When there is sufficient ATP and excess fuel, it will be dephosphorylated and more active.
What is the phosphorylation state of acetyl-CoA carboxylase when it is active?
palmitoyl-CoA (the final product of fatty acid synthesis) citrate - citrate is a signal that there is excess acetyl CoA
Acetyl-CoA carboxylase is allosterically inhibited by -------- and activated by ---------.
Acetyl-CoA carboxylase has three components: (1) a biotin carboxyl carrier protein; (2) biotin carboxylase, which adds CO2 to the prosthetic group; and (3) carboxyltransferase, which transfers the activated CO2 unit to acetyl-CoA. The long, flexible biotin-lysine chain (biocytin) enables the activated carboxyl group to be carried between the biotin carboxylase and the carboxyltransferase.
Describe the structure of acetyl-CoA carboxylase.
In Omega fatty acids, double bonds are counted from the methyl end, whereas in delta fatty acids, double bonds are counted from the carboxyl end.
Double bonds in unsaturated fatty acids are referred to as omega fatty acids or delta fatty acids.
2-carbon units acetyl-CoA
Fatty acid chains are made via addition of - carbon units derived from -------.
1. Intermediates in fatty acid synthesis are covalently linked to sulfhydryl groups of acyl carrier proteins (not the SH group of coenzyme A). 2. Fatty acid synthesis occurs in the cytosol (not the mitochondria). 3. Reactions are carried out by a single protein with multiple enzymatic activities - fatty acid synthase. 4. Fatty acid synthesis uses NADP+/NADPH as the coenzyme for oxidation-reduction reactions (not NAD+/NADH).
Fatty acid synthesis is not simply a reversal of beta-oxidation. What are the key differences?
triacylglycerides (TAGs) or membrane lipids
Free fatty acids do not remain in the cytosol, but rather are rapidly esterified and moved into
1. The acetyl group of one acetyl-CoA is transferred to ACP by MAT and next transferred to a cysteine sulfur group in the KS enzyme. 2. The malonyl group of malonyl-CoA is transferred to the newly opened ACP site by MAT. 3. Condensation of both two-carbon units to acetoacetyl-ACP is driven by the decarboxylation of malonyl-CoA. This is catalyzed by KS and occurs at the ACP site. 4. Acetoacetyl-CoA, (the four carbon unit) is reduced to hydroxybutyryl-ACP (KR), dehydrated to crotonyl-ACP (DH), and further reduced to butyryl-ACP (ER). 5. The processed four-carbon unit is transferred to the cysteine sulfur group in KS, and a new malonyl group binds the open ACP site.
Generally, what occurs in the reactions of fatty acid synthesis?
Malonyl-CoA, produced during fatty acid synthesis, inhibits the uptake of fatty acylcarnitine (and thus fatty acid oxidation) by mitochondria. When fatty acyl-CoA levels rise, fatty acid synthesis is inhibited and fatty acid oxidation activity increases. Rising citrate levels (which reflect an abundance of acetyl-CoA) similarly signal the initiation of fatty acid synthesis.
How are is the regulation of fatty acid synthesis and fatty acid oxidation coupled?
Unphosphorylated acetyl-CoA carboxylase binds citrate with high affinity and thus is active at very low citrate concentrations. Phosphorylation of the regulatory sites decreases the affinity of the enzyme for citrate, and in this case, high levels of citrate are required to activate the carboxylase. The inhibition by fatty acyl-CoAs operates in a similar but opposite manner. Thus, low levels of fatty acyl-CoA inhibit the phosphorylated carboxylase, but the dephosphoenzyme is inhibited only by high levels of fatty acyl-CoA. Specific phosphatases act to dephosphorylate ACC, thereby increasing the sensitivity to citrate.
How are the regulatory effects of citrate and palmitoyl-CoA dependent on the phosphorylation state of acetyl-CoA carboxylase?
Unsaturated fatty acids are produced by terminal desaturases. In the reaction catalyzed by fatty acyl-CoA desaturase, a double bond is formed between two carbon atoms and oxygen is reduced to water.
How are unsaturated fatty acids formed from a newly synthesized fatty acid?
1. First, acetate units are activated by the formation of malonyl-CoA. 2. The addition of two carbon units to the growing chain is driven by decarboxylation of malonyl-CoA. 4. The elongation reactions are repeated until the growing chain reaches 16 carbons in length (palmitic acid). 5. Other enzyme are used for elongation beyond 16 C and for desaturation.
What are the general steps of fatty acid synthesis?
Citrate activation and palmitoyl-CoA inhibition of acetyl-CoA carboxylase are strongly dependent on the phosphorylation state of the enzyme. This provides a crucial connection to hormonal regulation. Many of the enzymes that act to phosphorylate acetyl-CoA carboxylase are controlled by hormonal signals. Glucagon is a good example. Glucagon binding to membrane receptors activates an intracellular cascade involving activation of adenylyl cyclase. Cyclic AMP produced by the cyclase activates protein kinase A, which then phosphorylates acetyl-CoA carboxylase. Unless citrate levels are high, phosphorylation causes inhibition of fatty acid biosynthesis. The carboxylase (and fatty acid synthesis) can be reactivated by a specific phosphatase, which dephosphorylates the carboxylase. Additionally, it is important to note the simultaneous activation by glucagon of triacylglycerol lipases, which hydrolyze triacylglycerols, releasing fatty acids for β-oxidation. Both the inactivation of acetyl-CoA carboxylase and the activation of triacylglycerol lipase are counteracted by insulin, whose receptor acts to stimulate a phosphodiesterase that converts cAMP to AMP.
How do hormonal controls regulate fatty acid synthesis?
Bicarbonate is activated for carboxylation reactions by formation of N-carboxybiotin. ATP drives the reaction forward, with transient formation of a carbonylphosphate intermediate (Step 1). In a typical biotin-dependent reaction, nucleophilic attack by the acetyl-CoA carbanion on the carboxyl carbon of N-carboxybiotin—a transcarboxylation—yields the carboxylated product (Step 2).
How does the mechanism of the acetyl-CoA carboxylase reaction work?
eicosanoids
Locally acting hormones derived from C20-fatty acids like arachidonic acid. They molecules act locally as hormones, mediating inflammation, reproductive functions, pain and fever, blood clotting, blood pressure, and the sleep/wake cycle. They bind GPCRs but are not found in blood circulation. They only act in tissues in which they are produced. Ex) prostaglandins, prostacyclins, thrombaxanes, and leukotrienes are local mediators made from 20-carbon polyunsaturated fatty acids (such as arachodonic aicd).
Mammals cannot synthesize the omega3 or omega6 fatty acids, which are essential in our diets. These fatty acids are made by plants (and some fish since fish eat plants).
Mammalian desaturases introduce double bonds between the 8- or 9- position and the carboxyl end, or closer to the carboxyl end (delta9 or lower) - but not at positions closer to the methyl end. What are the consequences of this?
arachidonic acid, EPA, and DHA
Mammals cannot synthesize linoleic acid or linolenic acid, but when they are consumed, they can use these fatty acids as precursors for...
Glycerolipids, which use glycerol as a backbone. The two main classes are glycerophospholipids and triacylglycerols. Sphingolipids, which use sphingosine as a backbone. These may both be modified by carbohydrates or phosphates.
Membrane lipids include...
elongation reactions or desaturation reactions
Palmitate is the primary product of fatty acid synthesis. Following release of palmitate via the TE activity of FAS, ---------reactions and/or ---------- reactions may produce additional forms of fatty acids.
Phosphatidic acid is converted into either diacylglycerol or CDP-diacylglycerol. Diacylglycerol is a precursor for TAGs, phosphatidyl-choline and phosphatidyl-ethanolamine (and phosphatidyl-serine). CDP-diacylglycerol is a precursor for phosphatidyl-inositol, phosphatidyl-glycerol, and cardiolipin
Phosphatidic acid is the precursor for all other glycerophospholipids. How is this precursor converted to other molecules?
CDC-diacylglycerol
Phosphatidylinositol, phosphatidyl-glycerol, and cardiolipin are made from
phosphatidyl-ethanolamine
Phosphatidylserine is made from
fatty acid synthase
Protein dimer responsible which contains all reaction centers required to produce a fatty acid (6 enzymatic activities). 2 of these are condensing activities (MAT and KS) and 4 of these are modifying activities (KR, DH, ER, and TE).
acetyl-CoA
Source of 2-carbon units for elongation reactions IN THE MITOCHONDRIA
malonyl-CoA
Source of 2-carbon units for elongation reactions ON ER SURFACE
In our intestines, TAGs are remade from dietary TAGs. They are made by acylation of digestion products. In adipose and liver, TAGs are made from DAG (starting from glycerol or DHAP).
TAGs are made from different precursors in different tissues. Explain.
acyl carrier protein (ACP)
The basic building blocks of fatty acid synthesis are acetyl and malonyl groups, but they are not transferred directly from CoA to the growing fatty acid chain. Rather, they are first passed to ----------. This protein consists of a single polypeptide chain of 77 residues to which is attached (on a serine residue) a phosphopantetheine group, the same group that forms the "business end" of coenzyme A. Thus, it is a somewhat larger version of coenzyme A, specialized for use in fatty acid biosynthesis.
ACP
The flexible arm of --- transports substrates to different enzyme sites in fatty acid synthase
malonyl-CoA
The main source of two-carbon units during fatty acid synthesis
malonyl-CoA
The main source of two-carbon units during fatty acid synthesis.
sphingomylein galactosylcermaide Myelin is rich in both of these.
The major sphingolipid A major sphingolipid
arachindonic acid
The most prevalent eicosanoid precursor. It has 4 non-conjugated double bonds and is synthesized from linoleic fatty acid. In cells, it is found in cell membranes at the C2 position of phosphoglycerolipids. It must be released from phospholipids in response to some signal to be converted to the relevant eicosanoid.
PGH2
The precursor for all of the prostaglandins, which are important to responding to some type of infection.
Palmitoyl-CoA and serine
The precursors for ceramide
There are three principal sources for acetyl-CoA: 1. Amino acid degradation produces cytosolic acetyl-CoA. 2. Fatty acid oxidation produces mitochondrial acetyl-CoA. 3. Glycolysis yields cytosolic pyruvate, which (after transport into the mitochondria) is converted to acetyl-CoA by pyruvate dehydrogenase. ***primary source The acetyl-CoA derived from amino acid degradation is normally insufficient for fatty acid biosynthesis, and the acetyl-CoA produced by pyruvate dehydrogenase and by fatty acid oxidation cannot cross the mitochondrial membrane to participate directly in fatty acid synthesis. Instead, acetyl-CoA is linked with oxaloacetate to form citrate, which is transported from the mitochondrial matrix to the cytosol. Here it can be converted back into acetyl-CoA and oxaloacetate by ATP-citrate lyase. In this manner, mitochondrial acetyl-CoA becomes the substrate for cytosolic fatty acid synthesis. (Oxaloacetate returns to the mitochondria in the form of either pyruvate or malate, which is then reconverted to acetyl-CoA and oxaloacetate, respectively.) NADPH can be produced in the pentose phosphate pathway as well as by malic enzyme. Reducing equivalents (electrons) derived from glycolysis in the form of NADH can be transformed into NADPH by the combined action of malate dehydrogenase and malic enzyme. Thus, the citrate-malate-pyruvate shuttle provides cytosolic acetate units and some reducing equivalents (electrons) for fatty acid synthesis. The shuttle collects carbon substrates, primarily from glycolysis but also from fatty acid oxidation and amino acid catabolism.
Where do the NADPH and acetyl-CoA come from for fatty acid synthesis?
gangliosides
cerebrosides containing sialic acid
NSAIDS
class of molecules that blocks the cyclooxygenase activity of PGHS; in other words, they block the synthesis of PGH2, preventing inflammation and fever.