Lipids serve many functions: energy storage/source, fat soluble vitamins act as coenzymes, prostaglandins and steroid hormones are necessary for homeostasis
Digestion
Dietary fats consist mainly of triacylglycerols
Remainder consists of cholesterol, cholesteryl esters, phospholipids and free fatty acids.
Digestion is minimal in the stomach and mouth.
Upon entry into the duodenum, emulsification occurs, which is the mixing of two normally immiscible liquids
Emulsion formation increases the surface area of the lipid: permits greater enzymatic interaction and procession
Emulsification is aided by bile: bile salts, pigments and cholesterol
This is secreted by the liver and stored in the gallbladder
The pancreas secretes pancreatic lipase, colipase & cholesterol esterase into the small intestine as well.
Enzymes hydrolyze the lipid components into 2-monoacylglycerol, free fatty acids, and cholesterol
Micelle Formation
Emulsification is followed by absorption of fats by intestinal cells
Micelles: consist of free fatty acids, cholesterol, 2-monoacylglycerol, and bile salts
These are clusters of amphipathic lipids that are soluble in aqueous intestinal environment
Water soluble spheres with lipid soluble interior
Micelles are present from the duodenum all the way to the end of the ileum
Bile salts are recycled and restored at the end
Absorption
Micelles diffuse into the brush border of the intestinal mucosal cells
Digested lipids pass through the brush border and are absorbed into the mucosa and re-esterified to form triacylglycerols and cholesteryl esters
Chylomicrons: packages of triacylglycerols, cholesteryl esters, apoproteins, fat-soluble vitamins, and other lipids
Leave the intestine via lacteals (lactic system vessels), and then reenter the bloodstream via the thoracic duct (long lymphatic vessel that empties into the left subclavian vein)
Water soluble fatty acid chains can be absorbed by simple diffusion into the bloodstream.
Lipid Mobilization
Hormone-sensitive lipase (HSL): hydrolyzes triacylglycerols which yields fatty acids and glycerol
This is activated by a fall in insulin levels or an increase in cortisol and epinephrine
Released glycerol from fat may be transported to the liver for glycolysis or gluconeogenesis
HSL is effective within adipose cells
Lipoprotein Lipase (LPL): is necessary for the metabolism of chylomicrons and very-low-density lipoproteins (VLDL)
This is an enzyme that can release free fatty acids from triacylglycerols in these lipoproteins
Lipid Transport
Free fatty acids are transported through the blood in association with an albumin carrier protein
Triacylglycerol and cholesterol are transported in the blood as lipoproteins:
Aggregates of apolipoproteins and lipids
These are named after according to their density
Density increases in proportion to the percentage of protein in the particle
Chylomicrons are the least dense (high fat-to-protein ratio), followed by VLDL, followed by IDL, LDL and HDL
Chylomicrons
Highly soluble in lymphatic fluid and blood
Function in the transport of dietary triacylglycerols, cholesterol, and cholesteryl esters
Assembly occurs in the intestinal lining. A newly formed chylomicron that contains lipids and apolipoproteins
VLDL (Very-Low-Density Lipoproteins)
Metabolism is similar to chylomicrons
Difference is that VLDL is produced and assembled in the liver cells
Main function is the transport of triacylglycerols
Also contains fatty acid that are synthesized from excess glucose or retrieved from chylomicrons remnants
IDL (Intermediate-Density Lipoproteins)
The resulting protein from when a triacylglycerols is removed from the VLDL
Sometimes referred to as a VLDL remnant
Some IDL is reabsorbed by the liver by apolipoproteins and some is further processed in the bloodstream
Some pick up cholesteryl esters from HDL to become LDL
Exists as a transition particle between triacylglycerol transport and cholesterol transport
LDL (Low-Density Lipoproteins)
Majority of the cholesterol measured in blood is associated with LDL
Normal role is to deliver cholesterol to tissues for biosynthesis
Also important role in cell membranes and in the formation of bile salts
Also required for steroid hormone synthesis: steroidogenesis
HDL (High-Density Lipoprotein)
Synthesized in the liver and intestines and release as dense, proteins-rich molecules into the blood.
Contains apolipoproteins used for cholesterol recovery
I.e. – the cleaning up of excess cholesterol from blood vessels for excretion.
HDL can also deliver some cholesterol to steroidogenic tissues and can transfer necessary apolipoproteins to other lipoproteins
Apolipoproteins (apoproteins)
Receptor molecules that are involved in signaling
Do not need to know specific function of each apolipoprotein, just know that they are used for a diverse range of purposes.
Cholesterol Metabolism
Plays a role in the synthesis of cell membranes, steroid hormones, bile acids, and vitamin D
Sources
Derived mainly from LDL or HDL. May also be synthesized in the liver (de novo)
De novo synthesis is driven by Acetyl-CoA and ATP
Citrate shuttle carries mitochondrial Acetyl-CoA into the cytoplasm, where synthesis occurs
NADPH supplies the reducing equivalents (from the PPP)
Synthesis of mevalonic acid in the smooth endoplasmic reticulum is the rate-limiting step in cholesterol biosynthesis
Catalyzed by 3-hydroxy-3 methylglutaryl (HMG) CoA reductase
Synthesis of cholesterol is regulated in many ways:
Increased levels of cholesterol inhibit further synthesis
Insulin promotes cholesterol synthesis
De Novo synthesis is also dependent on the regulation of HMG-CoA reductase.
Specific Enzymes
Lecithin-cholesterol acyltransferase (LCAT): found in the bloodstream and is activated by HDL apoproteins
Adds a fatty acid to cholesterol. This produces soluble cholesteryl esters such as those in HDL
Cholesteryl ester transfer protein (CETP): facilitates the transfer of cholesteryl esters from HDL to other lipoproteins like IDL
Fatty Acid and Triacylglycerols
Fatty Acids: long chain carboxylix acids
Carboxyl carbon is carbon 1 and carbon 2 is known as the alpha-carbon
Nomenclature
Total number of carbons is given along with the number of double bonds
E.g. – carbons: double bonds
Can be further specified by indicating the position and isomerism of the double bonds
Saturated fatty acids: no double bonds while unsaturated fatty acids have one or more double bonds
Humans can only synthesize a few fatty acids, the rest come from diet
a-Linolenic Acid & Linoleic acid: poly unsaturated fatty acids are important in maintain cell membrane fluidity,
Omega (w) numbering system: used for unsaturated fatty acids
w designation describes the position of the last double bond in relation to the end of the chain
Also identifies the major precursor fatty acid
Double bonds are usually in the cis configuration
Synthesis
Fuel fatty acids are primarily supplied through diet, but excess carbohydrates and proteins can also be converted to fatty acids and stored as energy reserves
Lipid and carbohydrate synthesis is termed non-template synthesis
Since they do not rely on the coding of a nucleic acid
Fatty Acid Biosynthesis
This occurs in the livers, while its products are transferred to adipose tissue
Adipose also has the ability to synthesize smaller quantities of fatty acids
Synthesis is driven by the reactants: Acetyl-CoA carboxylase & fatty acid synthase
These are stimulated by insulin
Palmitic acid (palmitate): primary end product of fatty acid synthesis
Acetyl-CoA Shuttling
Acetyl-CoA accumulates in the mitochondrial matrix if a large fatty meal is eaten
It needs to be moved to the cytosol for fatty acid biosynthesis since it slows the citric acid cycle
Acetyl-CoA is a product of the pyruvate dehydrogenase complex
It combines with oxaloacetate to form citrate, this begins the citric acid cycle
Limited by isocitrate dehydrogenase
A buildup of acetyl-CoA causes a buildup of citrate
Citrate is able to diffuse across the mitochondrial membrane and be split up (by citrate lyase)
Splits into acetyl-CoA and oxaloacetate
Acetyl-CoA Carboxylase
This is the rate limiting step of fatty acid synthesis
Acetyl-CoA carboxylase requires ATP and biotin to function
Adds a CO2 to Acetyl-CoA to form malonyl-CoA
Enzyme is activated by insulin and citrate
CO2 is never incorporated into the fatty acid since it is removed during fatty acid synthase
Fatty Acid Synthase
More appropriately called palmitate synthase since palmitate is the only fatty acid that humans can synthesize de novo
Large multi-enzyme complex that is found in the cytosol
Induced by the liver after meals eaten that are high in carbohydrates (insulin induces the complex)
Complex contains an acyl carrier protein (ACP)
This protein requires pantothenic acid (Vitamin B5)
NADPH is required to reduce acetyl groups that are added to the fatty acid
Palminate requires 8 acetyl-CoA groups
Fatty acyl-CoA can be elongated and desaturated to a limited extent by the smooth endoplasmic reticulum
Steps involved are: attachment to acyl carrier protein, bond formation between activated malonyl-CoA and the growing chain, reduction of a carbonyl group, dehydration and reduction of a double bond
Reaction occurs many times until a 16 carbon palmitate is created.
Can usually be reversed by B-oxidation
Triacylglycerol Synthesis
Formed by attaching three fatty acids (as fatty Acyl-CoA) to glycerol
Storage form of fatty acids
Occurs primarily in the liver and a little in the Adipose tissue
Small amount also comes directly from the diet
In the liver, triglycerides are packaged and sent to adipose tissue as VLDL
Action of Fatty Acid Synthase:
a. activation of growing chain
b. malonyl-CoA
c. bond formation between activated molecules
d. Reduction of carbonyl to a hydroxyl group
e.. dehydration
f. reduction to a saturated fatty acid
Oxidation
Majority of fatty acid catabolism proceeds via b-oxidation
Occurs mainly in the mitochondria, but peroxisomal oxidation also can occur
Insulin inhibits indirectly while glucagon stimulates
Branched chain fatty acids undergo a-oxidation
w-oxidation occurs in the endoplasmic reticulum and produces dicarboxylic acids
Activation
When metabolized, fatty acids first become activated by attachment to CoA
Known as Fatty-Acyl-CoA synthetase or fatty Acyl-CoA or Acyl-CoA
Fatty Acid Entry into Mitochondria
Short chain (2-4 carbons) and medium chain (6-12 carbons) diffuse freely into mitochondria, where they can be oxidized
Long chain (14-20 carbon) require transport into the mitochondria via a carnitine shuttle
Carnitine acyltransferase I is the rate limiting enzyme of fatty acid oxidation
Very long chain fatty acids (over 20 carbons) are not oxidized within the mitochondria
Beta-Oxidation in Mitochondria
Reverses the process of fatty acid synthesis by oxidizing and releasing molecules of acetyl-CoA
Pathway is a repetition of four steps, and each cycle releases one acetyl-CoA
Also reduces NAD+ and FAD
FADH2 & NADH are oxidized in the electron transport chain to produce ATP
In muscle and adipose tissue, acetyl-CoA enters the citric acid cycle
In the liver, acetyl-CoA stimulates gluconeogenesis through activation of pyruvate carboxylase
When fasting, the liver produces more acetyl-CoA from beta-oxidation than is used up in the citric acid cycle
A lot of the acetyl-CoA is used to form ketones bodies that are released into the bloodstream and transported to other tissues.
Four steps of oxidation:
Oxidation of fatty acid to form a double bond
Hydration of the double bond to form a hydroxyl group
Oxidation of the hydroxyl group to form a carbonyl (beta-ketoacid)
Splitting of the beta-ketoacid into a shorter acyl-CoA and acetyl-CoA
Odd-numbered fatty acids have a different final cycle as compared to the norm
Even numbered fatty acids yield two acetyl-CoA
Odd numbered fatty acids yield one acetyl-CoA and one propionyl-CoA
Propionyl-CoA can be converted to glucose (i.e- only odd numbered fatty acids can be converted to glucose in humans)
Propionyl-CoA is converted to methylmalonyl-CoA by propionyl-CoA carboxylase
Requires biotin: Vitamin B7
Methylmalonyl-CoA can then be converted to succinyl-CoA by methylmalonyl-CoA mutase. This steps requires cobalamin (Vitamin B12)
Succinyl-CoA is used in the citric acid cycle as an intermediate, but can also be converted to malate to enter the gluconeogenic pathway in cytosol.
Unsaturated fatty acids require two additional enzymes to account for the double bonds
Enzyme must have at most one double bond in their active site
Bond must be located between carbons 2 & 3
Enoyl-CoA isomerase rearranges cis double bonds at the 3,4 position to trans double bonds at the 2,3 positions
This is the only extra step required so that monounsaturated fatty acids can undergo beta-oxidation
Polyunsaturated fatty acids require further reduction by using 2,4-dienoyl-CoA reductase
This converts two conjugated double bonds to just one double bond at the 3,4 position
Will then undergo the same rearrangement as above
Ketone Bodies
When fasting, the liver converts excess acetyl-CoA (from beta oxidation) into ketone bodies acetoacetate and 3-hydroxybutyrate (b-hydroxybutyrate)
This can be used for energy in various tissues
Cardiac and skeletal muscle, and the renal cortex are able to metabolize above two products back into acetyl-CoA
If fasting lasts for longer than a week, concentration of ketones in blood becomes high enough to allow the brain to begin metabolizing them
Ketogenesis
Occurs in the mitochondria of liver cells when excess acetyl-CoA accumulates in the fasting state
HMG-CoA synthase forms HMG-CoA
HMG-CoA lyase breaks down HMG-CoA into acetoacetate
Acetoacetate can be reduced to 3-hydroxybutyrate and acetone (is not used for energy)
Ketolysis
Acetoacetate is picked up from the blood and is activated in the mitochondria by succinyl-CoA acetoacetyl-CoA transferase (thiophorase)
Thiophorase is an enzyme that is only present in tissues outside of the liver
Liver lacks the enzyme so that it does not consume the ketones it produces
3-hydroxybutyrate is oxidized to acetoacetate
Ketolysis in the Brain
After one week of fasting, brain derives two-thirds of its energy from ketone bodies
When ketones are metabolized to acetyl-CoA, pyruvate dehydrogenase is inhibited
This switch spares essential proteins which would ordinarily be used in gluconeogenesis to produce glucose.
Protein Catabolism
Protein is rarely used as a source of energy since it is so important for other functions
Only occurs in cases of extreme energy deprivation
Proteolysis
The breakdown of proteins
Begins in the stomach with pepsin and continues with the pancreatic enzymes: trypsin, chymotrypsin and carboxypeptidases A/B
Secreted as zymogens (inactive form that is activated by something else)
Protein digestion is completed in the small intestine by its brush-border enzymes: dipeptidase and aminopeptidase
End products are amino acids, dipeptides, and tripeptides
Absorption of these products through the luminal membrane is accomplished through secondary active transport that is linked to sodium
Once at the basal membrane, amino acids can be transported to the bloodstream through simple and facilitated diffusion
Protein for energy can be used from the diet or from the body (during starvation)
Body protein is catabolized in the muscle and liver
Amino acids that are released from proteins usually lose their amino group through transamination or deamination
Remaining carbon skeleton is then used for energy
Amino acids can be classified by their ability to turn into metabolic intermediates:
Glucogenic: can be converted to glucose through gluconeogenesis
Include all amino acids except leucine and lysine
Ketogenic: can be converted into acetyl-CoA and ketone bodies
Includes leucine, lysine, isoleucine, phenylalanine, threonine, tryptophan and tyrosine
Amino groups that are removed through transamination must be disposed of safely since they can potentially be toxic
Urea Cycle: primary method of removing excess nitrogen from the body
