Mcat biochem Carbohydrates PDF

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Professor: Dr. Heather Tienson-Tseng
MCAT Carbohydrates...

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Ch 5 | Carbohydrates and Metabolism by Oxidation Saturday, April 10, 2021

1:53 PM

Mono + Disaccharides -

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Monosaccharides have general formula CnH2nOn ○ Two monosaccharides=disaccharide § Bound by a glycosidic linkage (covalent bond) formed by dehydration rxn Glycosidic linkages named using the configuration of the linkage and the carbons holding it ○ Lactose: Gal-B-1,4-Glc ○ Sucrose: Glc-a-1,2-Fru ○ Maltose: Glc-a-1,4-Glc ○ Cellobiose: Glc-B-1,4-Glc Alpha- the oxygen on the anomeric carbon points down Beta- the oxygen on the anomeric carbon points up ○ Remember: Cardi B is Up

Polysaccharides -

Glycogen: glucose joined in a-1,4 linkages and some a-1,6 ○ Starch is the same Cellulose is made of Beta glycosidic bonds that allow it to assume a long, straight, fibrous conformation Hydrolysis into monosaccharides is thermodynamically favorable ○ Different enzymes are specific for hydrolysis of glycosidic linkages (ie. Maltase, lactase) § Enzymes are so specific that they recognize stereochemistry □ Mammalian enzymes cannot digest Beta linkages (lactose)

Glucogenesis -

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A process in the liver where non-carbohydrate precursor molecules are converted into glucose ○ These molecules include lactate, pyruvate, Krebs cycle intermediates, amino acid carbon skeletons Occurs when dietary sources of glucose are unavailable and the liver has depleted

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its stores of glycogen 11 step pathway using many glycolysis enzymes - essentially glycolysis in reverese ○ PFK, hexokinase, and pyruvate kinase are not included (they catalyze irreversible rxns)

Steps 1. 2 CO2 added to 2 pyruvate, converting into 2 oxaloacetates a. Uses 2 ATP b. Enzyme: pyruvate carboxylase 2. 2 oxaloacetates are decarboxylated and phosphorylated into 2 phosphoenolpyruvate (PEP) a. Uses 2 GTP b. Expels 2 CO2 (favorable process to drive unfavorable) c. Enzyme: PEP carboxykinase (PEPCK) 3. 2 PEP converted to 2 2-phosphoglycerate 4. 2 2-phosphoglycerate into 2 3-phosphoglycerate 5. 2 3-phosphoglycerates phosphorylated by 2 ATP into 2 1,3-bisphosphoglycerate 6. Combined into 1 molecule of glyceraldehyde 3-phosphate a. Converts 2 NADH + 2 H into 2 NAD+ 7. Glyceraldehyde 3-phosphate converted to fructose-1,6-bisphosphate 8. Fru-1,6-bisP phosphate group removed to form fructose-6-phosphate a. Enzyme: fructose-1,6-bisphosphatase 9. Fructose-6-phosphate isomerized into glucose-6-phosphate 10. Glu-6-P dephosphorylated into glucose a. Enzyme: glucose-6-phosphatase Summary: four ATP and 2 GTP and 2 NADH required - All these high energy phosphate bonds make it thermodynamically favorable □

Glycolysis and glucogenesis pathways are regulated to avoid futile cycling- wasting energy by running them simultaneously ○ Reciprocal control- same molecule regulates two enzymes in opposite ways § PFK and fructose-1,6-bisphosphatase are both heavily regulated because they play opposite roles □ Both are allosterically regulated by intermediates that activate one enzyme and inhibit another ® High AMP levels activate PFK and inhibit F-1,6-Bpase ® High ATP levels would activate glucogenesis instead § Fructose-2,6-bisphosphate is a molecule that regulates both molecules and its levels are controlled by insulin and glucagon li 26 i l d i d l l i

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Insulin presence > F-2,6-BP stimulated > PFK activated > glycolysis performed □ Glucagon would trigger its breakdown Pathway prediction rules: ○ Enzymes that catalyze irreversible exergonic reactions in the pathway are most regulated ○ Increased concentration of pathway intermediates decrease the activity of that pathway ○ Cellular respiration stimulated by energy deficits (high ADP or high NAD+) and inhibited by energy surpluses (high ATP/NADH)

Glycogen Metabolism -

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Glycogenesis is the formation of glycogen (NOT TO BE CONFUSED WITH GLUCOGENESIS) ○ Starts with glucose-6-phosphate § Reversible reaction catalyzed by phosphoglucomutase converts it into Glucose-1-P □ Activated with UTP to form UDP-Glucose with UDP-glucose pyrophophorylase ® This is added to glycogen chain using the UDP and glycogen synthase Glycogenolysis- removal of glucose from glycogen polymer ○ Done by phosphorylating a unit at the end of the polymer, essentially reverse of the glycogenesis pathway Both of these occur in liver during fasting state and skeletal muscle during exercise ○ Skeletal muscle lacks glucose-6-phosphatase in order to keep glucose phosphorylated and unable to leave the muscle cell Stimulated and inhibited opposingly ○ Insulin stimulates glycogenesis when blood glucose is high, glucagon stimulates glycogenolysis when blood glucose is low

Pentose Phosphate Pathway -

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PPP- also known as the hexose monophosphate shunt ○ Diverts glucose-6-phosphate away from glycolysis to form NADPH, ribose-5phosphate, and glycolytic intermediates Occurs in cytoplasm Irreversible oxidation phase (makes NADPH and ribose-5-P) followed by a series of

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Irreversible oxidation phase (makes NADPH and ribose 5 P) followed by a series of reversible reactions to make glycolytic intermediates NADPH is a reducing agent used in anabolic processes like fatty acid synthesis and neutralization of radicals Ribose-5-P is used in nucleotide synthesis Glycolytic intermediates will be shunted back into glycolysis This pathway is regulated by regulation of the first enzyme in the pathway ○ Glucose-6-phosphate dehydrogenase (G6PDH) § NADPH acts as negative feedback on the enzyme § Deficiency in this enzyme limits ability of RBCs to eliminate reactive oxygen radicals

Cellular Respiration -

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Oxidation of glucose and reduction of high-energy electron carries NAD+ and FAD ○ NAD+- nicotinamide adenine dinucleotide § Activates adenylate cyclase ○ FAD- flavin adenine dinucleotide § Can associate with proteins to make flavoproteins that are involved in redox rxns ○ In their reduced forms, they are FADH2 and NADH § FADH2 results in less ATP production ○ NAD+ and FAD are cofactors (see roles above) Basic Steps: 1. Glucose is oxidized and NAD+ and FAD are reduced i. Electron carriers accept high energy electrons 2. NADH and FADH2 carry and deliver high energy electrons to electron transport chain, and are oxidized to NAD+ and FAD 3. Proton gradient is generated to power ATP synthesis

Step 1: Glycolysis Basics: - Glucose is partially oxidized and split in half into two pyruvic acid molecules - Small quantity of 2 ATP and 2 NADH produced - Occurs in cytoplasm - Anaerobic - All living cells perform it and possess the enzymes for it Steps: 1. Phosphate taken from ATP is used to phosphorylate glucose into Glucose 6phosphate (G6P)

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Enzyme: hexokinase i. Inhibited by G6P feedback G6P isomerizes into fructose F6P F6P is phosphorylated with another ATP into fructose-1,6-biphosphate (F1,6bP) 1. Enzyme: phosphofructokinase (PFK) i. Allosterically inhibited by ATP 2. This step is thermodynamically very favorable, so it's basically irreversible i. Key step in glycolysis - committed step F1,6bP is split into two 3-carbon units of Gde3P (aldehyde) Gde3P phosphorylated with Pi, producing 1,3bPGate (carboxyl) and 2 NAD+ reduced to 2 NADH + 2 H+ 1,3bPGate donates phosphate groups to ADP to generate 2 ATP and 3PGate Phosphate group shifts, forming 2PGate 2PGate converted to PEP PEP converted to pyruvate, 2 ADP converted to ATP 1. Enzyme: pyruvate kinase

*Step 1b: Fermentation Basics: - Anaerobic conditions - Occurs so that NAD+ can be regenerated so glycolysis can continue - Pyruvate is used as electron acceptor from NADH ○ Can be reduced to ethanol or lactate - Cori Cycle: the process by which liver deals with excess lactate from muscle after exercise ○ Lactate is converted back to pyruvate and NADH is generated from NAD+, and then the pyruvate and NADH are used in cellular resp.

Step 2: Pyruvate Dehydrogenase Complex Basics: - Pyruvate is decarboxylated to form acetyl group in a oxidative decarboxylation reaction - Small amount of NADH produced - Only occur when oxygen is present but does not use oxygen itself - Catalyzed by three complexed enzymes (efficient) - Occurs in mitochondria matrix - Pyruvate is converted from three carbon molecule to a 2-carbon acetyl unit and CO2, which is removed - Acetyl unit is activated (aka attached) to a carrier called coenzyme A (CoA-SH) and

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is known as acetyl-CoA when bound Prosthetic group- a cofactor tightly or covalently bound to an enzyme ○ PDC has TPP as a prosthetic group at one of its active site § TPP = thiamine pyrophosphate □ This is why we need thiamine Vitamin B

Step 3: Krebs/TCA Cycle Basics: - Acetyl group added to oxaloacetate to form citric acid - Citric acid decarboxylated and isomerized back to oxaloacetate - 2 turns of the cycle per one molecule of glucose (one turn for each acetyl-CoA supplied) - 6 NADH, 2 FADH2, and 2 GTP produced per glucose - Only occur when oxygen is present but does not use oxygen itself - Occurs in mitochondria matrix Steps: 1. Acetyl Co-A enters and is combined with oxaloacetate to form 6-carbon Citrate 1. Citrate contains three carboxylic acids and gives TCA cycle its name 2. Citrate is oxidized into a 5-carbon intermediate (a-ketoglutarate) one CO2 (released) and one NADH 3. A-ketoglutarate is oxidized to produce 4-carbon intermediate (succinyl-CoA), one CO2 and one NADH 4. Oxaloacetate is regenerated, and 1 NADH and 1 FADH2 are produced, as well as a high-energy phosphate bond in GTP

Step 4: Oxidative Phosphorylation Basics: - NADH and FADH2 dump their electrons into the electron transport chain when they are oxidized - Oxygen is reduced at the end of the chain into H2O - Electron energy is used to pump protons out of the mitochondrial matrix against gradient - Protons flow back into the mitochondria and this energy drives ATP synthesis - Occurs in the inner mitochondrial membrane - Electron transport chain consists of 5 electron carriers that reduce each other in a line ○ Three are large cytochrome proteins w/ heme group A, B, and C ○ The other two are small mobile carriers, Q and CytoC Goals: - Reoxidation of all electron carriers from glycolysis PDC and TCA cycle to

Reoxidation of all electron carriers from glycolysis, PDC, and TCA cycle to regenerate NAD+ and FAD - Storage of energy in ATP in the process Steps: 1. (A) NADH dehydrogenase receives electrons from NADH and oxidizes it to NAD+ 2. A passes its electrons onto (Q) ubiquinone which received electrons from FADH2 and oxidizes it to FAD 1. Pumps out protons in process 3. Ubiquinone passes its electron to (B) Cytochrome C reductase 4. B passes its electrons to cytochrome C, which is small and hydrophilic, pumps out protons 5. Cytochrome C passes its electrons to (C) cytochrome C oxidase 6. C passes its electrons to oxygen, which is reduced to H2O 1. Pumps out protons in the process 7. High pH inside the matrix (and low pH in intermembrane space) due to proton gradient causes protons to flow through ATP synthase in membrane 1. ATP is synthesized from ADP and Pi -

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Quantitative Outcomes: - NADH is responsible for pumping of 10 protons each - 4 protons = 1 molecule ATP - 1 NADH = 2.5 ATP - FADH pumps 6 protons across membrane - 1.5 ATP = 1 FADH Prokaryotes can still perform oxidative phosphorylation even though they lack mitochondria ○ They do it on the cell membrane ○ Because they don't have to shuttle around electron carriers since everything is in the cytosol, they yield 2 more high energy phosphate bonds from the process In eukaryotes, glycerol phosphate shuttle brings glycolysis electron transporters to the ETC ○ Delivers electrons directly to ubiquinone and bypasses NADH dehydrogenase, so it only results in 1.5 molecules of ATP per cytosolic NADH ○ Thus, prokarya produce more ATP per glucose Actual yield of ATP per glucose depends on proton availability for ATP synthesis Current understanding is that 30 ATP result from eukaryotic cellular metabolism and 32 ATP for prokaryotes...