Biochemistry 3 Flashcards by Alegra Mendez

isoprenoids

A class of lipids.
Ex: cholesterol

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long chain fatty acids

> 12 carbons.

Absorbed in small intestine.

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medium chain fatty acids

6-12 carbons

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short chain fatty acids

<6 carbons.
Produced in colon by microbiome.
Can cross blood-brain barrier.
6-10% of total energy.

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Essential FAs

Linoleic Acid: omega 6, [18:2 delta 9,12]
Linolenic Acid: omega 3 [18:3 delta 9,12,15]

Deficiency: alopecia, scaly dermatitis, thrombocytopenia, cognitive development, dermatitis

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TAGs (general properties)

3 FA chains.
Glycerol backbone.
Ester bond.

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Leptin

Suppresses food intake.
Released by adipose.
Alters hormones released from hypothalamus

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Ghrelin

Increases food intake.

“Hunger hormone”

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digestion of lipids (before pancreas)

Mouth - lingual lipase.
Stomach - pancreatic lipase.

Both important for short/medium chain FA synthesis in neonates and those with pancreatic insufficiency.

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digestion of lipids (after stomach)

Pancreas:
Pancreatic lipase - hydrolyzes esters 1 and 3, forming 2 FFA and 1 MAG.
Factor colipase: binds pancreatic lipase to keep it at the lipid-aqueous interface.
Cholesterol esterase: hydrolyzes cholesterol esters.
Phospholipase A2: removes 1 phospholipid FA.
Lysophospholipase: removes 2nd FA.

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bile salts

Cholesterol derivatives.
Made in liver.
Secreted from gall bladder.
Conjugated with glycine/taurine (negative charge).
Release controlled by CCK.
80-95% recycled via enterohepatic circulation.

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gall stones caused by

imbalance of cholesterol, bile, and phospholipid secretion

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pancreatitis

300,000 hospital admissions per year.
Elevated serum lipase and amylase.
Nausea, vomiting, pain, steatorrhea.

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elevated bile salts

Causes pruritis.

Treatment: ursodeoxycholic acid.

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entry of digested lipids into enterocytes

Micelles bring cholesterol, FFA, and MAG to the enterocyte membrane (short/med FFA don’t need micelle).
Mainly enter via passive diffusion.
Cholesterol enters via NPC1 L1 transporter (receptor mediated).

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dietary cholesterol enters the enterocyte via

NPC1 L1 transporter (receptor mediated)

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export out of enterocytes

FFAs must be converted to CoA derivatives.
CoA derivatives + MAG = TAG (via TAG synthase).
Cholesterol converted to a cholesterol ester (via ACAT).
Chylomicrons form and are released into lacteal.

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chylomicron formation

Formed between lipid bilayer of ER membrane, then buds.
ApoB48 is added (now a nascent chylomicron).
MTP (microsomal TAG transfer protein) loads with TAGs and other lipids.

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chylomicron release

Nascent chylomicrons transported from ER to Golgi.
Vesicles from golgi fuse with plasma membrane and release into the lacteal.
Enter lymphatic system.
ApoC-II and ApoE transferred from HDL (now a mature chylomicron).
Enter blood through thoracic duct, subclavian vein.

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chylomicron degradation

Lipoprotein lipase activated by ApoC-II.
Adipose absorb resulting FAs and MAGs (passive).
ApoE and ApoB48 bind LRP, allowing chylomicron remnants to be internalized by the liver.
Endocytosis/degradation by liver.
Cholesterol esterase releases free cholesterol from cholesterol esters.

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LPL isozymes

Muscle LPL has a much higher affinity (lower Km) than adipose LPL.

Muscle needs energy basically all the time.
Adipose only needs FAs for storage.

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Bassen-Kornzweig Disease/ Abetalipoproteinemia

Genetic defect of MTP.
Low TAG/cholesterol levels.
Steatorrhea, abdominal distension.
Clumsiness, progressive ataxia, neuropathy, vision impairment.
High levels of acanthocytes (>50%)

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Familial chylomicronemia syndrome

No LPL –> cannot release FAs from chylomicrons.
Severe hypertriglyceridemia.
Pancreatitis.
Xanthoma.

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VLDL

Synthesized in liver.
TAGs and CEs added via TAG synthase and ACAT.
ApoB100 is a characteristic lipoprotein.
Exported in blood to adipose and other tissues.

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VLDL hydrolysis

Via LPL (lipoprotein lipase).
Stimulated by ApoC-II.
LPL is on capillary walls.

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IDL

When CE = TAGs.
ApoC-II falls off.
HTGL (Hepatocyte Triacylglycerol Lipase) removes remaining TAGs –> becomes LDL.

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LDL

Major transporter of cholesterol to tissues.
Longest half-life.
Can get oxidized if it circulates for too long (contributes to atherosclerosis.

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LDL oxidation

Oxidized LDL stimulates monocyte recruitment to blood vessel wall.
Monocytes differentiate into macrophages, and engulf ox-LDL, become foam cells, then die.
Atherosclerotic plaque forms (fatty streak is dead foam cells, smooth muscle growth, matrix degradation.

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HDL lifecycle

“Good cholesterol”.
Synthesized in liver.
Empty shell of phospholipids.
Characteristic lipoproteins: ApoA-I and ApoA-II.
ApoC and ApoE also present, and passed to chylomicrons/VLDL in the blood.
ApoAI stimulates LCAT: removes a FA from membrane to form CE from excess cholesterol that cells moved to their surface; produces lysolecithin.
Full HDLs are bound by SR-B1 receptors on liver, adrenals, for degradation.

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CHD risk factors (and effect on HDL/LDL)

Smoking decreases HDL.
Diabetes/obesity increases LDL.
Lack of exercise increases LDL, decreases HDL.

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Familial Hypercholesterolemia

Type of hyperlipoproteinemia.
Defective LDL receptor.
LDL stays in blood, gets oxidized.

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release of FAs stored in adipose

Glucagon and epinephrine activate cAMP-dependent protein kinase –> phosphorylates HSL –> hydrolyzes adipose TAGs to FAs.

FFAs transported on serum albumin.
Passively transported into cells for oxidation.

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prep for FA catabolism

Fatty acyl CoA synthetase activates FFA–> acyl CoA.

Activated long chain FAs need carnitine transporter to get into mitochondrial matrix.

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Carnitine Transporter

Long chain FAs added to carnitine via CPT1.
Fatty acylcarnitine is transferred into the mitochondrial matrix via carnitine:acylcarnitine translocase, in exchange for 1 carnitine exiting mito matrix.
CPT2 converts acylcarnitine to acyl-CoA in matrix.

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regulation of carnitine transporter

CPT1 inhibited by MCoA (1st step of FA synthesis)

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FA oxidation

Occurs on B-carbon.

1) Acyl CoA DH: forms 2,3-trans-enoyl CoA, produces FADH2
2) 2-3 Enoyl CoA Hydratase: Forms 3-hydroxyacyl CoA, requires water.
3) 3-OH-acylCoA DH: Forms 3-keto fatty acyl CoA, produces NADH.
4) 3-ketoacylCoA thiolase: Forms fatty acyl CoA + acetyl CoA
5) REPEAT

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net reaction of 1 cycle of FA oxidiation

1 FADH2
3 NADH
1 GTP
=
12 ATP

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fatty acyl CoA DH isozymes

1st step of B-oxidation.
VLCAD: 14-20 carbons
LCAD: 12-18 carbons (palitate, stearate)
MCAD: 6-12 carbons
SCAD: <6 carbons

unsaturated FA (extra steps in B-ox)

2,4 Dienoyl CoA Reductase: reduces 1 double bond, requires NADPH.
Enoyl CoA isomerase: rearranges double bond so B-ox can continue.

branched chain FAs

Phytanic acid, derived from plants.
Alpha-oxidation, thiolase, beta-oxidation, thiolase.
Produces propionyl CoA.

Refsum Disease

Genetic deficiency in alpha-oxidation (branched chain FAs).

Neurological disorder.

location of FA synthesis

cytosol

location of FA degradation

mitochondria

citrate shuttle

Citrate (from TCA) can exit mitochondria into cytosol.
In cytosol, can regenerate ACoA + oxaloacetate.
ACoA is then used for FA synthesis;
Oxaloacetate –> malate –> pyruvate.
Pyruvate enters mitochondria and repeats.
Produces NADPH.

preparation of ACoA for FA synthesis

ACoA –> MCoA
By enzyme ACC (acetyl CoA carboxylase).
Biotin dependent.
Requires ATP.

global regulation of ACC

ACC catalyses ACoA –> MCoA.

Epinephrine/glucagon activate PKA –> inhibits PP2A –> ACC remains inactive/phosphorylated.
Epinephrine/glucagon activate AMPK –> inactivates/phosphorylates ACC.

Insulin activates PP2A –> ACC becomes dephosphorylated/active

local regulation of ACC

Citrate (substrate) activates ACC into polymer form.

Absence of citrate deactivates ACC (monomer form).

FA Synthase (FAS)

Multiple enzymes in a single polypeptide.
Domain 1: substrate entry, condensation.
Domain 2: reduction.
Domain 3: palmitate release.

FA synthesis: situating substrates on enzyme

ACoA binds to CE (condensince enzyme) via ACoA Acetyltransferase.
MCoA covalently binds to ACP (Acl carrier protein) via MCoA-ACP Transacetylase.
ACP has phosphopantetheinyl group: flexible arm that moves substrates to each domain.

FA synthesis: reactions on enzyme

1) CONDENSATION:
Add MCoA to ACoA.
By CE (condensing enzyme) / 3-ketoacyl-ACP synthase.
Decarboxylates malonate, transferring 2 carbons to ACoA.
Releases CO2.
Forms acetyl-acetyl-ACP

2) REDUCTION
By B-ketoreductase / 3-ketoacyl-ACP reductase.
Uses NADPH.
Reduces ketone to hydroxyl.
Forms hydroxybutyryl-ACP.

3) DEHYDRATION
By dehydratase / 3-hydroxyacyl-ACP dehydratase.
Creates a double bond.
Releases water.
Forms Crotonyl-ACP.

4) REDUCTION
By Enoyl-ACP reductase.
Uses NADPH.
Reduces double bond.
Forms Butyryl-ACP

5) REPEAT

6) RELEASE PALMITATE
By thioesterase.
Requires water (hydrolysis).
*different thioesterase in mammary glands that releases C8-C14 FA which are more readily absorbable.

Net reaction of FA synthesis

8 ACoA + 7 ATP + 14 NADPH + 14 H+ —> Palmitate + 8 CoA + 6 H2O + 7 ADP + 7 Pi

general steps of FA synthesis (no enzymes/structures)

Attach MCoA to ACP.
Attach ACoA to CE.
Condensation.
Reduction.
Dehydration.
Reduction.
Repeat.
Release palmitate.

Step 1 of FA synthesis: condensation

1) CONDENSATION:
Add MCoA to ACoA.
By CE (condensing enzyme) / 3-ketoacyl-ACP synthase.
Decarboxylates malonate, transferring 2 carbons to ACoA.
Releases CO2.
Forms acetyl-acetyl-ACP

Step 2 of FA synthesis: reduction

2) REDUCTION
By B-ketoreductase / 3-ketoacyl-ACP reductase.
Uses NADPH.
Reduces ketone to hydroxyl.
Forms hydroxybutyryl-ACP.

Step 3 of FA synthesis: dehydration

3) DEHYDRATION
By dehydratase / 3-hydroxyacyl-ACP dehydratase.
Creates a double bond.
Releases water.
Forms Crotonyl-ACP.

Step 4 of FA synthesis: reduction

4) REDUCTION
By Enoyl-ACP reductase.
Uses NADPH.
Reduces double bond.
Forms Butyryl-ACP

release of palmitate after FA synthesis

RELEASE PALMITATE
By thioesterase.
Requires water (hydrolysis).
*different thioesterase in mammary glands that releases C8-C14 FA which are more readily absorbable.

elongation of FA

In smooth ER (sometimes in mitochondria).
Same steps as FA synthesis, but different enzymes.
Acyl chain carrier is CoA instead of ACP.
Elongates in units of 2 at carboxyl end.

desaturation of FAs

In smooth ER.
4 types of human desaturases: delta 4, 5, 6, 9.
Cannot desaturate past C9 –> linoleic and linolenic acids are essential.
By mixed function oxidases.
NADH, cytochrome b5, O2.

Modification of MUFA to PUFA

Add additional double bond.

By Fatty Acyl CoA Desaturase.

source of NADPH

ACoA synthesis from citrate shuttle (8 NADPH).

Pentose Phosphate Pathway (6 NADPH).

TAGs (general info)

3 FAs + glycerol backbone.
Ester bonds.
Position 1: saturated
Position 2: unsaturated
Position 3: either (usually saturated).

Is major storage in cytosol of adipocytes.

source of glycerol backbone for TAGs

1) glycerol from breakdown of TAGs
2) DHAP from glycolysis

Enzyme needed for option 1 is not present in adipocytes.
Adipocytes must use DHAP in adipose.

TAG synthesis

1) DHAP –> glycerol-phosphate
By glycerol-3-phosphate DH.

2) add FAs 1 and 2
By Acyl transferase.
Must first be activated by Acyl CoA synthetase.

3) get rid of the phosphate on the 3 position.
By phosphatidic acid phosphatase.
Requires water.
Forms a diacylglycerol.

4) add 3rd FA
By acyl transferase / diacylglycerol transferase (DGAT).
Must first be prepped by Acyl CoA synthetase.
Now have a complete TAG

DGAT is committed step

ketone bodies

Alternative fuel source.
Form after days of fasting, or in diabetics.
Water soluble.
Cross BBB.
Not used in liver.

3 forms of ketone bodies

acetone
acetoacetate
hydroxybutyric acid

location of ketone body synthesis

Mitochondria of liver.

why do ketone bodies form?

Excess of FAs delivered to liver.
More ACoA than liver needs is formed.
Converts excess ACoA into ketone bodies for export.

synthesis of ketone bodies

1) Condensation of 2 ACoAs
By tholase.
Forms Acetoacyl CoA.

2) Add another ACoA.
By HMG-CoA reductase.
Requires water.
Forms HMG-CoA.

3) Release 1 CoA.
By HMG-CoA lyase.
Forms acetoacetate.

*) Can make acetone from acetoacetate.
By acetoacetate decarboxylase.
Releases CO2.

*) Can make D-B-hydroxybutyrate from acetoacetate.
By hydroxybutyrate DH.
Uses NADH.

degradation of ketone bodies

In extrahepatic tissues.

1) Oxidation of hydroxyl to carbonyl.
Hydroxybutyrate –> acetoacetate.
Produces NADH.

2) Add CoA.
From succinyl-CoA (TCA cycle).
By ketoacyl-CoA transferase.

3) Add another CoA.
From CoA-SH.
By thiolase.
Forms 2 ACoA.

ACoA enters TCA cycle.

ketone bodies and uncontrolled diabetes

Ketone bodies can accumulate to high levels.
KBs are buffered with cations in blood (KBs are negatively charged).
When excreted, cations are excreted too.
With less cations, buffering ability of blood is reduced.
Ketoacidosis results.
Can be severe/fatal.

medical uses of ketogenic diets

PDH deficiency.
Epilepsy.
Type II DM : keeps glucose at a lower level; eliminates spikes in blood sugar, reducing need for insulin.

three major classes of membrane lipids

1) Glycerophospholipids (GPLs)
2) Phosphosphingolipids
3) Sphingolipids/glycolipids

glycerophospholipids (general)

Position 1: saturated FA
Position 2: unsaturated FA
Head alcohol group linked by phosphodiester bond.
Scaffold: glycerol.
Head groups: ethanolamine (E), choline (C), serine (S), glycerol (G), inositol (I)

In specific cases, di-saturated FAs are present to increase membrane fluidity (in lung alveoli).

phosphospingolipids (general)

Scaffold: sphingosine (1 built in FA)
Position 2: fatty acid
Head group alcohol linked by phosphodiester bond.
FA attached via AMIDE bond.
Important in brain, common membrane component.

sphingolipids/glycolipids

Scaffold: sphingosine (1 built in FA)
Position 2: FA
Head group: sugar (glucose/galactose).
FA attached via AMIDE bond.

major intermediateds/sources of GPL scaffold

GPL scaffold: glycerol

1) phosphatidic acid: from G3P in glycolysis.
2) DAG: from dietary fat

Phosphodiester Bond Formation on GPL

First the phosphate group (of either the head or scaffold, depending) is activated by linking CMP (cytidine monophosphate) to generate CDP (cytidine monophosphate).

If the DAG is activated with CDP, then head group hydroxyl can attack phosphate and bind (For PI, PG, Cardiolipin)
If head group is activated with CDP, then the hydroxyl on the scaffold can attack to make the bond (PE, PC)

When the hydroxyl attacks, it displaces CMP, forming the phosphodiester linkage.

Interconversion between Glycerophospholipids

PC: DAG + CDP choline or PE + 3xSAM methylation

PE: DAG + CDP ethanolamine or PS + decarboxylation of serine

PS: PE + exchange serine for ethanolamine

Phospholipases (PLs)

PLA1 cut off the FA from position 1.
PLA2 cut off FA from position 2.
PLCs cut to give DAG + the phosphorylated head group alcohol (decapitate).
PLDs give phosphatidic acid (DAG with a phosphate on C3) + the head group alcohol (decapitate, but leave phosphate).

PLA1/2

Cut off the FA from position 1 and 2, respectively.
PLAs can rearrange FAs.
PLAs are in many venoms.

Lysophospholipids (missing 1 FA) act as hemolytic detergents.

PLC

Cut to give DAG + the phosphorylated head group alcohol (decapitate).
Gives DAG + IP3 (signaling molecules)

PLD

Give phosphatidic acid (DAG with a phosphate on C3) + the head group alcohol (decapitate, but leave phosphate)

cyclooxygenases

Cyclooxygenase adds 2x O2 to PUFAs, causing loss of 2 double bonds)
Forms prostaglandins and thromboxanes.
Inhibited by aspirin and anti-inflammatory drugs.

prostaglandins and thromboxanes

Mediate pain sensation, inflammation, fever, thrombosis.

leukotrienes

Immune response signalling molecules.
Formed when PLA2-released PUFAs are acted upon by lipoxygenases.
Add just 1 O2, do NOT change # of double bonds.

Cellular chemotaxis, cytokine release, mediate vascular permeability/bronchocontriction.

DAG

Formed by cleavage of PIP2 by PLC (forms DAG and IP3).

Activates protein kinase C.

IP3

Formed by cleavage of PIP2  by PLC (forms DAG and IP3).
In cytoplasm.
Causes release of Ca++ from ER stores.
Activates calmodulin kinase.
Helps activate PKCs.

PLC cleavage of PIP2

Activated by G protein when peptide hormone binds to a membrane-spanning receptor.
Forms DAG and IP3.
Products are second messengers that allow the signal to be amplified inside the cell

Ceramide formed by

addition of 1 FA to sphingosine (through amide bond)

sphingomyelin formed by

addition of phosphodiester-linked choline to ceramide.

Myelin sheath component

galactocerebroside formed by

Addition of galactose to ceramide

Tay-Sachs disease

Defect in hexosamidase A.

Normally cuts a sugar off of ganglioside GM2 to generate GM3

cholesterol synthesis

Substrates: cytoplasmic ACoA and NADPH (same as FA synthesis).
HMG-CoA synthase makes HMG-CoA in cytoplasm.
HMG-CoA reductase reduces HMG-CoA to mevalonate (uses NADPH)–COMMITTED STEP.
Mevalonate converted

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Biochemistry 3 Flashcards

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