Lecture 13 (2-28) Flashcards
Two sources of fat for energy source
- Diet
- Fat stores (adipocytes)
Fat from Adipose Cells: how it is released/what happens in the beginning
- releasing Triacylglycerols (TAGs)
- Hormones (glucagon, epinephrine, ACTH) trigger the release of fatty acids from adipose tissue (adipocytes) - amplification/cascade - cascade allows you to release FAs for breakdown
- Enzymatic cleavage:
- —- TRIACYLglycerol
- —- set-up for DIACYLglycerol lipase and then MONOACYLglycerol lipase
- FA into circulation on HSA (human serum albumin), a protein in the blood - transports them to tissues that need them for energy production (FAs hop on them –> move through circulation until dropped off when needed)
Fat from Diet: breakdown within the body
- TAGs again (like fat from adipose cells)
- Some FA’s released in stomach but not extensively - acidic environment not optimal
- Small intestine is major site:
- —- pancreatic juices increase the pH
- —- mainly because of a high [bicarbonate ion]
- TAG breakdown
- —- pancreatic lipases
- —- non-specific esterases
- Enzymatic activity depends on bile salts
Note: A diverse array ofgenetically-distinct lipase enzymes are found in nature; and represent several types of protein foldsand catalytic mechanisms. Most are built on an alpha/beta hydrolase fold*and employ amechanism using a catalytic triadconsisting of a serine nucleophile, a histidinebase, and an acidresidue (usually Asp).
Fat from Diet: Bile Salts (definition, how they work) + later stage of digestion of dietary fats (what happens to fat after bile salts)
Bile salts: charged (carboxylic) steroids
- They emulsify fats (NOT micellar as shown in diagram)
Bile salts act in several ways:
- decrease surface tension of fat globules
- increase area accessible to enzymes
- mobilize end products away from globules as micelles
FA’s that are:
- short chain enter villi directly
- med or long chain - micellized into mixed micelles
- —- condensed with glycerol -> TAGs
- —- carried to blood through lymphatics
- In small intestine, FAs combine with bile salts in mixed micelles, which deliver FAs to epithelial cells that cover the intestinal villi. TAGs are formed within the epithelial cells.
Beta-Oxidation of Fatty Acids: scientific experiments
Knoop fed dogs FA with a terminal phenyl group:
- ‘Even numbered FA’ yielded phenyl acetate
- Odd numbered FA yielded benzoate
- Therefore, fatty acids must be degraded by removal of 2-C units
- VERY CLEVER EXPERIMENT!!!
Lehninger showed that this occurred in the mitochondria
- Knoop assumed that 2-C until were acetate - close but not correct
- Lynn/Reichart: showed that the 2-C unit released was acetyl-CoA
- The process begins with oxidation of the carbon “Beta” to the carbonyl carbon (hence, “Beta-oxidation”)
Beta-Oxidation of Fatty Acids: the Players (and their location)
Acyl-CoA synthetase and Carnitine acyl transferase are critical
- on outer part of inner mito membrane
All the rest: IN the mito matrix
Beta-Oxidation of Fatty Acids: the role of CoA
CoA activates Fatty Acids for Oxidation
Acyl-CoA synthetase condenses fatty acids with CoA, with simultaneous hydrolysis of ATP -> AMP + PPi
- formation of a CoA ester is expensive energetically
Reaction just barely breaks even with ATP hydrolysis
- SO what drives it?? Le Chatelier!
Beta-Oxidation of Fatty Acids: the role of Carnitine
Carnitine is a carrier
- Carnitine key to transport of long chain fatty acyl groups across the inner mitochondrial membrane
Long-chain fatty acids:
- can’t be directly transported
- converted to acyl carnitines, then then transported
- A three protein system coordinates the net transfer of long-chain acyl-CoA esters into the mito
Carnitine as a supplement
- Sources: meats and fish
Supposedly aids in:
- weight loss: NOPE (a study of women showed no difference between weight loss in those who took carnitine and those who didn’t - some experienced nausea, diarrhea)
- brain function: YES (studies indicate that taking acetyl-L-carnitine daily helps reverse the decline of brain function associated with Alzheimer’s and other brain diseases. This form exhibited similar benefits for general brain function in older adults lacking Alzheimer’s or the brain conditions)
- heart health: YES (some studies demonstrate a potential for reducing blood pressure and the inflammatory process associated with heart disease)
Beta-Oxidation of Fatty Acids: basics, reactions (why they are familiar), products, strategy, what is happening (chemistry) in fourth reaction
- repeating a 4-step sequence
- strategy: create a carbonyl group on the beta carbon
First three reactions:
- dehydrogenation
- hydration
- oxidation
- Where have we seen this before??? TCA cycle (succinate dehydrogenase, fumarate, malate dehydrogenase)
Fourth reaction: CLEAVE the “beta-keto ester” (reverse Claisen condensation)
Products: an acetyl-CoA and a fatty acid shorter by 2C’s
Beta-Oxidation: Reaction 1
Acyl-CoA Dehydrogenase
- Oxidation of the Ca-Cb bond
- First of two oxidations
- A complex - three soluble matrix enzymes - differing specificity
- Mechanism involves proton abstraction, followed by double bond formation and hydride removal by FAD
- Electrons are passed to an electron transfer flavoprotein
Fatty acyl-CoA + FAD —> FADH2 + trans-delta-Enoyl-CoA
Beta-oxidation connection fo ETS
- direct connection to ETS
- Fatty acyl-CoA dehydrogenase -> flavoprotein -> UQ/UQH2 pool
Acyl-CoA dehydrogenase
beta oxidation
- oxidation of the Ca-Cb bond
- Electrons are passed to an electron transfer flavoprotein, and then to the electron transport chain - note ATP yield
Enoyl-CoA Hydratase
beta oxidation
- adds water across the double bond
- At least three forms of the enzyme are known
- termed “crotonases”
- normal reaction converts trans enoyl-CoA to L-b-hydroxyacyl-CoA (stereospecific manner)
Hydroxyacyl-CoA Dehydrogenase
beta oxidation
- oxidizes the beta-hydroxyl group
- 2nd oxidation: specific for L-hydroxyacyl-CoA
- Note: NADH produced –> 2.5 ATP
Hydroxyacyl-CoA Dehydrogenase
beta oxidation
- oxidizes the beta-hydroxyl group
- 2nd oxidation: specific for L-hydroxyacyl-CoA
- Note: NADH produced –> 2.5 ATP
L-beta-hydroxyacyl-CoA + NAD+ –> NADH + H+ + Beta-Ketoanyl-CoA
Beta-Oxidation: Reaction 4
beta oxidation
- cleavage of 2-C by Thiolase
- Thiols key in the cleavage of beta-ketoacyl-CoA
- This is the reverse Claisen condensation: attack of the enblate or acetyl-CoA on a thioester
- Even though it forms a new thirster, the reaction has favorable Keq and drives other three
- Cysteine thiolate on enzyme key
Beta-ketoacyl-CoA + CoA-SH –> Acyl-CoA + acetyl-CoA?
Summary of Beta-Oxidation: overview, energy yield
- Repetition of the cycle yields a succession of acetyl CoA’s
- Recall the ATP yield from 1 glucose: 32-38 ATP
- For palmitic acid (C16), oxidation yields: 8 Acetyl CoA’s, 7 NADH, 7 FADH2
- Complete beta-oxidation of the AcCoA’s from palmitic acid yields 106 molecules of ATP
- Again, large energy yield is consequence of:
- —— the highly reduced state of the carbon
- —— the highly concentrated storage capability with FA
Beta-Oxidation vs. Glucose
- In beta-oxidation, 106 ATP produced (vs. 38 ATP)
- 6.33 ATP/C (ATP) vs. 6.63 ATP/C (beta-oxidation)
This makes fatty acids the fuel of choice for:
- migratory birds -> energy (migrate without food or bathroom breaks)
- the Plover flies 1800 miles non-stop!
