Lectures 19/20: Carbohydrate Metabolism Flashcards by Sabrina Secord

Negative deltaG

Keq greater than 1
More products than substrates in equilibrium
Exergonic reaction towards products
Favourable reaction towards products

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Positive deltaG

Keq is less than 1
More substrate than products in equilibrium
Endergonic reaction towards products
Non-favourable reaction towards products

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GLUT

Specific glucose transporters that take glucose inside cell
Several forms based on tissue and cell type
Transporters facilitate bidirectional transport of glucose (in and out), always from higher to lower concentration of glucose
Does not transport phosphorylated glucose

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Glucose uptake

By GLUT
Reversible, deltaG nearly 0
Direction of glucose transport depends on substrate/product levels
Phosphorylation removes glucose from equilibrium
Entry of glucose depends on GLUT transporters and the activity of hexokinase

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Pyruvate

Glucose is converted to 2 pyruvate, 2 3-carbon molecules

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Glycolysis

Oxidation of glucose to pyruvate
Net yield of 2 ATP
2 ATP are invested, and 4 are made
Electron carriers are reduced

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Gluconeogenesis

Reverse conversion of pyruvate to glucose
Reversible glycolysis reactions use the same enzyme
Irreversible glycolysis reactions use different enzymes

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Phase 1 of glycolysis

Energy investment
Steps 1-5
Phosphorylation of glucose and conversion of 2 molecules of glyceraldehyde-3-phosphate
Two ATP are used

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Phase 2 of glycolysis

ATP production phase
Steps 6-10
Conversion of glyceraldehyde-3-phosphate to pyruvate and coupled formation of 4 ATP
Reduction of 2NAD+ to 2NADH

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Step 1 of glycolysis

Hexokinase phosphorylates glucose to glucose-6-phosphate
1 ATP used
Irreversible

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Step 2 of glycolysis

Isomerization of glucose 6-phosphate to Fructose-6-phosphate

Catalyzed by phosphoglucose isomerase (PGI)

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Step 3 of glycolysis

Phosphorylation of Fructose-6-phosphate to Fructose-1,5-bisphosphate
1 ATP used
Irreversible
Catalyzed by phosphofructokinase

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Phosphofructokinase-1

Phosphorylates fructose-6-phosphate to give fructose-1,6-phosphate (more symmetrical)
Allosterically regulated by fructose-2,6-BP
Addition of ATP reduces PFK1 and more F-2,6-BP needed to activate
Addition of AMP increases activity

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Steps 4 and 5 of glycolysis

Cleavage of carbon backbone to dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phsohate (GAP)
Isomermization of DHAP and GAP by triose phosphate isomerase

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Triose phosphate isomerase

Isomerizes GAP and DHAP (become readily interchangeable, allows glycolysis to proceed using the same enzymes for each)

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Step 6 of glycolysis

Oxidation and addition of inorganic phosphate to GAP by glyceraldehyde-3-phosphate dehydrogenase
NAD is needed
1,3-biphosphateglycerate (1,3-BPG) and NADH are produced

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Glyceraldehyde-3-phosphate dehyrogenase

Oxidized and adds phosphate to GAP

Generates 1,3-bisphosphoglycerate and NADH

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Step 7 of glycolysis

Dephosphorylation and first generation of ATP from 1,3-BPG by phosphoglycerate kinase to generate 3-phosphoglycerate
Direction and flux influenced by ATP

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Step 8 of glycolysis

Phosphoglycerate mutate moves phosphate from 3 to 2 position on 3-phosphoglycerate

Phosphoglycerate kinase

Generates 3-phosphoglycerate from 1,3-BPG

Generates 1 ATP (but occurs twice per glucose molecule)

Step 9 of glycolysis

Dehydration of 2-phosphoglycerate to phosphoenolpyruvate by enolase
Generates H2O

Steps 10 of glycolysis

Formation of pyruvate and generation of second ATP
Irreversible
Catalyzed by pyruvate kinase

Pyruvate kinase

Catalyzes the generation of ATP and pyruvate from phosphoenolpyruvate
Happens twice per glucose molecule to give 2 ATP total

Pyruvate

Can take many different routes
Aerobic: mitochondrial conversion of pyruvate to acetyl-CoA and oxidation in the TCA cycle
Anaerobic: cytosolic regeneration of NAD+

Alcoholic fermentation

Yeast regenerates NAD+ by making ethanol | Catalyzed by pyruvate decarboxylase and alcohol dehydrogenase

Lactic fermentation

Pyruvate and NADH fermentation by lactase dehydrogenase gives lactic acid and NAD+ Lactate is secreted from the cell and acidifies the environment

Anaerobic glycolysis

Glycolysis followed by conversion of pyruvate to lactate Generates 2ATP Full oxidation of pyruvate to CO2 requires oxygen and mitochondria

Gluconeogenesis

Generation of glucose from various substrates: pyruvate, lactate, glycerol, most amino acids, all citric acid cycle intermediates Not exact reverse of glycolysis Unique gluconeogenic enzymes: glucose phosphatase, fructobisphosphatase, phosphoenolpyruvate carboxykinase, pyruvate carboxylase

Glucose phosphatase

Reverse of hexokinase Glucose phosphorylation by hexokinase is irreversible and has high -deltaG If both were coupled, it would have a net cost of 1ATP

Oxaloacetate

For last step reversal (first step of gluconeogenesis from pyruvate) pyruvate carboxylase catalyses oxaloacetate formation from pyruvate Requires ATP

Pyruvate carboxylase

Converts pyruvate to oxaloacetate, using 1ATP

Phosphoenolpyruvate carboxykinase

Converts oxaloacetate to phosphoenolpyruvate | Uses 1GTP

Phosphofructokinase 2

Phosphorylates fructose-6P at 2 carbon to generate Fructose-2,6-BP

Fructose-2,6-bisphosphate

Most potent activator of phosphofructokinase in mammals Allosteric activator of PFK1 Inhibits fructobisphosphatase: inhibits gluconeogenesis

Product inhibition

Product of enzyme inhibits enzyme | Does not change reaction, but changes rates

Covalent modification

Usually catalyzed by other enzyme | Usually phosphorylations and dephosphorylations through kinases and phosphatases

Allosteric control

Feedback, feedforward within one pathway Metabolites from other pathways regulate connected pathways Feedforward interaction is more rare

Feedback inhibition

Prevents overproduction of product

Feedforward activation

Ensures completion of the pathway

Pentose pathway

Provides different intermediates Intermediates of pathway can be used for synthesis of nucleotides Highly adaptable to needs of cell Feeds into glycolysis Can make 2 fructose-6-P and glyceraldehyde-3-phoshate (GAP)

Glycogen

Synthesized from monomer of glucose-1-phosphate - isomerized from glucose-6-phoshate

UTP

Bonds with glucose-1-P to give glucose-UDP and 1P | Glucose added to glycogen polymer though glycogen synthase and release of UDP

Glycogen break down

Linear chaines broken down via phosphorolysis | Branched chains broken down by hydrolysis

Fructose metabolism

Usually phosphorylated by hexokinase to fructose-6-phosphate Does not occur in liver No feedback mechanism If liver gets overwhelmed by byproducts, they can enter fat synthesis Liver transports can only transport a fraction of fructose

Fructose metabolism in liver

Glucoinase cannot metabolize fructose | Converted to fructose-1-phosphate by fructokinase

Fructose-1-Phosphate

converted to glyceraldehyde-3-phosphate which enters glycolysis after main regulatory step