BIO 311 Lecture 8: Fatty Acids and Amino Acids to Acetyl CoA

Digestion of TAG, and roles of liver, gallbladder and pancreas in lipid digestion.

• Digestion of TAG

  • Bile salts emulsify dietary fats in small intestine => mixed micelles

  • Intestinal lipases (from pancreas) degrade triacylglycerols

  • Fatty acids and other breakdown products are taken up by intestinal mucosa and converted into triacyclglycerols

  • Triacylglycerols are incorporated, w/ cholesterol and apolipoproteins, into chylomicrons

  • Chylomicrons move through lymphatic system and bloodstream to tissues

  • Lipoprotein lipase, activated by apoC-II in the capillary, converts TAG to fatty acids and glycerol.

  • Fatty acids enter cells

  • Fatty acids are oxidized as fuel or reesterified as storage

• Liver: produces bile salts

• Gallbladder: stores bile

• Pancreas: provides intestinal lipases

Names of the abundant 12C, 14C, 16C and 18C saturated fatty acids; and the three essential fatty acids commonly present in diet.

• 12C

  • Common name: Laurate

  • Systematic name: n-Dodecanoate

• 14C

  • Common name: Myristate

  • Systematic name: n-Tetradecanoate

• 16C

  • Common name: Palmitate

  • Systematic name: n-Hexadecanoate

• 18C

  • Common name: Stearate

  • Systematic name: n-Octadecanoate

• Essential fatty acids: Linoleate (18C), Linolenate (18C), Arachidonate (20C)

Lipid soluble vitamins and general understanding their roles in human biology/physiology.

• Lipid soluble vitamins

  • G(K), D, A, E

• Roles:

  • Vision, bone health, immune function, coagulation

Clinical consequence of malabsorption of fatty acids.

Deficiency of lipid soluble vitamins, essential fatty acids, and dehydration

General structure of chylomicron, the role of B-48 protein.

    

• Single layer of phospholipids -> polar heads facing water

• Triacylglycerols sequestered in the interior make up more than 80% of mass

• Many apolipoproteins that protrude from surface (B-48, C-III, C-II) act as signals in uptake and metabolism of chylomicron contents

• Diameter ranges from about 100 to 500 nm

• B-48 role: surface protein marker that identifies itself and regulates uptake

fatty acids activation in cytosol

• Transport or attachment to phospholipids requires conversion to fatty acyl-CoA

• Fatty acid -> Fatty acyl-CoA

  • Enzymes: fatty acyl-CoA synthetase and inorganic phosphatase

  • Highly exergonic reaction

  • Total: 2 ATP equivalents consumed

    • Consume 1 physical ATP molecule, but produce AMP

    • To return AMP back to ATP, use 2 inorganic phosphates (1 ATP equivalent)

Translocation to mitochondria + Rate Limiting Step

• Beta oxidation of fatty acids occurs in mitochondria

• Fatty acids may be transported as free fatty acids or more commonly, by lipoproteins (chylomicrons)

• Small (< 12 carbon) fatty acids diffuse freely across mitochondrial membranes

• Larger fatty acids (most free fatty acids) are transported via acyl-carnitine/carnitine transporter

• RATE LIMITING STEP OF FATTY ACID OXIDATION

  • After fatty acyl-carnitine is formed at the outer membrane or in intermembrane space, it moves into mitochondrial matrix by facilitated diffusion through the transporter in the inner membrane

  • In matrix, acyl group is transferred to mitochondrial coenzyme A (CoA-SH), freeing carnitine to return to intermembrane space via same transporter

  • Carnitine acyltransferase I (CAT I): outer mitochondrial membrane

  • Carnitine acyltransferase II (CAT II): inner mitochondrial membrane

  • Transporter: inner membrane

Carnitine

• Carnitine = ID Card

• Not a vitamin; synthesized by human body from lysine

• Synthesis depends on vitamin C

• Fatty acyl group: transferred to OH group => ester

Oxidation of Fatty Acids

• Stage 1 (Beta oxidation): a long-chain fatty acid is oxidized to yield acetyl residues in the form of acetyl-CoA

• Stage 2: the acetyl groups are oxidized to CO₂ via citric acid cycle

• Stage 3: electrons derived from oxidations of stages 1 and 2 pass to O₂ via mitochondrial ETC, providing energy for ATP synthesis by oxidative phosphorylation

The 4 step reactions in the repeating rounds of beta-oxidation.

• first three steps follows pattern similar to steps 6-8 of TCA (dehydrogenate then hydrate then dehydrogenate)

• Step 1: Dehydrogenation of Alkane to Alkene

  • Enzyme: isoforms of acyl-CoA dehydrogenase (AD) on the inner mitochondrial membrane

    • FAD cofactor

    • Very-long-chain AD (12-18 C)

    • Medium-chain AD (4-14 C)

    • Short-chain AD (4-8 C)

  • Result: trans-enoyl-CoA; trans double bond, different from naturally occurring unsaturated fatty acids

  • Analogous to succinate dehydrogenase reaction in the citric acid cycle

    • Electrons from bound FAD transferred directly to the ETC via electron-transferring flavoprotein (ETF)

• Step 2: Hydration of Alkene (SAYS THIS IS ENOL HOW?)

  • Enzyme: two isoforms of enoyl-CoA hydratase

    • Soluble short-chain hydratase (crotonase)

    • Membrane-bound long-chain hydratase, part of trifunctional complex

  • Result: beta-hydroxy-acyl-CoA; water adds across double bond => alcohol on beta carbon

  • Analogous to fumarase reaction in the citric acid cycle

  • Same stereospecificity

• Step 3: Dehydrogenation of Alcohol

  • Enzyme: beta-hydroxyacyl-CoA dehydrogenase

    • NAD cofactor

  • Result: Beta-ketoacyl-CoA

  • Only L-isomers of hydroxyacyl CoA act as substrates

  • Analogous to malate dehydrogenase reaction in the citric acid cycle

• Step 4: Transfer of Fatty Acid Chain and Release of Acetyl-CoA

  • Enzyme: acyl-CoA acetyltransferase (thiolase) via covalent mechanism

  • Result: thiolysis of the carbon-carbon bond

    • Carbonyl carbon in beta-ketoacyl-CoA is electrophilic

    • Active site thiolate acts as a nucleophile and releases acetyl-CoA

    • Terminal sulfur in CoA-SH acts as a nucleophile and picks up the fatty acid chain from the enzyme

Given an even numbered fatty acids or fatty acyl-CoA, point out the end products, including FADH2 and NADH, from beta oxidation and the amount of each.

• # of FADH₂/NADH₂ formula (check)

  • (Even # / 2) – 1 = # of FADH₂/NADH

  • Ex: Palmitic acid (16C) => 7 FADH₂ and 7 NADH

Given an even numbered fatty acid or fatty acyl-CoA, if completely oxidized, the amount of ATP production with functional ETC.

• # of ATP formula:

  • (Even # / 2) – 1) * 4 ATP = # of total ATP from beta oxidation rounds

  • (Even # /2) * 10 ATP = # of total ATP from ETC

  • Sum them up

  • NET ATP: Consider the two ATP reduced in cytosol

How to catabolize unsaturated and odd-numbered fatty acids? Cofactor needed for catabolism of propionyl-CoA?

• Oxidation of Unsaturated Fatty Acids

  • We can do it!

  • Naturally occurring unsaturated fatty acids contain cis double bonds

    • NOT a substrate for enoyl-CoA hydratase

• Two additional enzymes required

  • Isomerase: converts cis double bonds starting at carbon beta to trans double bonds

  • Reductase: reduces cis double bonds not at carbon beta

• Monounsaturated Fatty Acids:

  • Require isomerase

  • Shift double bond, cis to trans using enoyl-CoA isomerase

  • 1 Acyl-CoA dehydrogenase step skipped => 1 less FADH₂ produced

• Polyunsaturated Fatty Acids:

  • Require both enzymes (isomerase and reductase)

  • We can do it!

  • Less energy yield

  • Involves NADPH dependent reductase

• Oxidation of Odd-Numbered Fatty Acids

  • We can do it!

  • Most dietary fatty acids are even-numbered (b/c of fatty acid synthesis), but many plants and some marine organisms synthesize odd-numbered fatty acids => our diets

  • Propionyl-CoA: 3 carbon compound that forms during final cycle of beta oxidation of odd-numbered fatty acids

    • Some amino acids also produce propionyl-CoA

  • Oxidation of Propionyl-CoA pathway:

    • When beta oxidizing odd-numbered fatty acids, you eventually get to propionyl-CoA

    • Propionyl-CoA -> D-Methylmalonyl-CoA

      • Enzyme: propionyl-CoA carboxylase

      • Cofactor: biotin

    • D-Methylmalonyl-CoA -> L-Methylmalonyl-CoA

      • Enzyme: methylmalonyl-CoA epimerase

    • L-Methylmalonyl-CoA -> Succinyl-CoA

      • Enzyme: Methylmalonyl-CoA mutase

      • Cofactor: Coenzyme B12 (cobalamin)

    • Send to TCA Cycle

    • VITAMIN B12 DEPENDENT

      • Most complicated B family vitamin

      • Cobalt ion in the middle

      • B12 is synthesized by microbials (need animal source diet -> egg, fish, etc.)

Consequence of defective medium chain fatty acyl CoA dehydrogenase.

• 1/10000 births

• Defects in fatty acids utilization

• Causes hypoglycemia

• Causes sudden infant death syndrome (10% of cases)

Roles of stomach and pancreas in protein digestion and three types of enzymes involved in protein digestion.

• Protein digestion start in stomach via pepsin enzymes => cleaves peptide bonds, reducing large polypeptides to smaller polypeptides

• Small polypeptides move to small intestine via peristalsis, where proteases (synthesized in enzyme) break them down into short polypeptides (tripeptides, dipeptides), and individual amino acids

• Enzymes in small intestine epithelium eventually break the short polypeptides into amino acid monomers

General pattern of amino acids catabolism to form metabolites of Krebs cycle

• Removal of Amino group from amino acid => alpha-keto acid ; transfer amino group to alpha-ketoglutarate => glutamate

• Fates of alpha-keto acid x:

  • Directly a metabolite of TCA cycle

  • ndirectly converted to a metabolite of TCA cycle

• If generate ketone body or Acetyl-CoA: ketogenic

• If not: glucogenic (important in carbohydrate metabolism)

• Catabolism of one amino acid may synthesize another one

How to remove non-alpha amino/amine groups?

• Majority: Alpha amino groups -> Transamination

  • Amino group from alpha-amino acid transferred to alpha-ketoglutarate to form alpha-keto acid X and glutamate, respectively

• Other:

  • Non-alpha amine (glutamine, asparagine, arginine) -> Hydrolysis

  • -OH groups (Serine, threonine) -> Dehydration-Initiated Deamination

  • Glutamate/Glutamic acid in Urea Cycle -> Oxidative Deamination

  • Other pathways to TCA

Transamination reaction, aminotransferase and cofactor needed. Vitamin precursor?

• Enzyme: aminotransferase

• Cofactor: PLP (pyridoxal phosphate)

• Precursor: vitamin B6

• Reversible reaction

• Involves either:

  • Alpha-ketoglutarate (amino acceptor)

  • Glutamate (donor)

• Things to know about vitamin B6

  • Transamination always needs vitamin B6

  • Amino acid metabolism (ex: decarboxylation of amino acid) needs vitamin B6

  • Vitamin B6 participates in various reactions

Overall correlation between common organic compounds including ketoacids (2C-5C) and corresponding amino acids precursors.

• 2C: Acetyl-CoA (ketogenic -> you do not increase carbons in TCA cycle -> not glucogenic -> not anaplerotic)

• 3C: Propionyl-CoA (3C) to Succinyl-CoA (glucogenic)

• 4C: Oxaloacetate (glucogenic)

• 5C: Alpha-Ketoglutarate (glucogenic)

The conserved pattern of oxidation of reduced carbon molecules.

• TCA cycle, beta-oxidation, and amino acid oxidation follow same pattern same sequence: DEHYDROGENATION -> HYDRATION -> DEHYDROGENATION

• Dehydrogenation b/t alpha-beta generate double bond and FADH₂

• After those common steps, structure differences determine further steps of metabolism.