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Glucose Metabolism

Terms in this set (54)

What is glycolysis?
- pathway made up of 10 enzymes-catalyzed steps in which glucose (C6H12O6) is transformed into two molecules of pyruvate (C3H3O3)
- Anerobic process, therefore doesn't require O2
- Usually the same in every cell but differs based on regulation of the pathway
- have net yield of 2 ATP and 2 NADH
What are the 2 stages of glycolysis?
- phase 1: preparatory (cost 2 ATP, consists of 5 steps where glucose is phosphorylated and spilt into 2 triode phosphates)
- phase 2: Payoff ( 5 steps in which oxidation and phosphorylation yield 2 NADH and 4 ATP)
What is the energy balance of glycolysis
- exergonic reaction
- 146 kJ/mol are released
- complete oxidation of glucose releases 2840 kJ/mol of energy and only 5.2% of this is released during the process
Step one of glycolysis
- Glucose is made into glucose-6-phosphate by doing a kinase transfer using one ATP to transfer the terminal phosphate to an acceptor
- mg2+ and ATP are needed as cofactors
- endergonic reaction
Step 2 of glycolysis
- glucose-6-phosphate to fructose-6-phosphate
- Glu-6-P is isomerized to Fructose-6-P via aldose/ketone isomerization
- unfavourable step, needs energy
Step 3 of glycolysis
- Fructose 6-phosphate --> Fructose 1,6-bisphosphate
- Uses phosphofructokinase enzyme (PFK) and changes ATP --> ADP
- exergonic step driven by ATP
- PFK
what is PFK
- allosteric enzyme and key control point in glycolysis
- activated by AMP and inhibited by ATP at allosteric binding sites sites
- sensor that can detect the energy state of the cell
What is the rational for allosteric enzyme
- since the purpose of of glycolysis is to synthesize ATP, ATP can be regarded as the end product of biosynthetic pathway for which PFK catalyzes an early step
- if ATP levels rise in cell, it will be useful inhibitor to prevent more production by feedback mechanism
- high levels of AMP mean that the cell needs to make more energy
What does adenylate kinase do?
- as ATP is used, its converted to ADP, and this kinase can squeeze a bit more energy out of ADP, well converting it to AMP
Step 4 of glycolysis
- Fructose 1,6-bisphosphate <--> dihydroxyacetone + glyceraldehyde 3-phosphate.
- essentially 6 carbon converted to 2 3carbon molecules
- unfavourable but pulled forward by removal of products in the following steps and overall free energy released by entire pathway .
Step 5 of glycolysis
- Dihydroxyacetonephosphate <--> glyceraldehyde 3-phosphate
- Uses triose phosphate isomerase enzyme.
- now 3, Glu-3-P making two products each for the next steps
- unfavourable
Step 6 of glycolysis
- Glyceraldehyde 3-Phosphate + Pi <--> 1,3-biphosphoglycerate.
- Uses G3P dehydrogenase enzyme.
- NAD+ <--> NADH
- oxidation and phosphorylation (aldehyde is oxidized to an acid and free energy is released to be used to reduce NAD+ to from high energy phosphate (acyl phosphate) that conserves 49.3 kJ/mol
- NAD+ is consumed and needs to be regenerated for glycolysis to continue
- energy conserving step
-
Step 7 of glycolysis
- 1,3-bisphosphoglycerate + ADP + H2O <--> 3-phosphoglycerate + ATP
- Uses phosphoglycerate kinase enzyme
- Energy realized to for ATP
- substrate level phosphorylation occurs where phosphorylation of ADP --> ATP occurs with direct involvement of a phosphorylated substrate
- one ATP produced per 3-C so 2 ATP produced per glucose
How is 7 performed when its exergonic
- uses products of step 6 by coupling rxn
- form channeling where the products of one are passed directly to the active site of the next enzyme
Step 8 of glycolysis
- 3-phosphoglycerate <--> 2-phosphoglycerate
- Uses phosphoglycerate mutase enzyme
- rearrangement with intramolecular transfer of phosphate group
- mutase tranfers Pi to an enzyme than to C2
What is a mutase?
an enzyme that catalyzes the transfer of a functional group from one position to another on the molecule
Step 9 of glycolysis
- 2-phosphoglycerate <--> Phosphoenolpyruvate (PEP)
- Uses enolase enzyme.
- Dehydration reaction (loss of water) that causes redistribution of energy in the molecule (phosphate trapped in unstable enol form) and G' is for hydrolysis of phosphate group PEP greatly increased overall of 2-PG
- second reaction that generate high energy intermediate
Step 10 of glycolysis
- PEP + ADP --> Pyruvate + ATP
- Uses pyruvate kinase enzyme
- very spontaneous
- second substrate level phosphoylation
- pyruvate is allosterically inhibited by ATP, acetyl-coA, and fatty acids
What is lactic acid fermentation?
- in muscle, NAD+ is consumed faster than TCA cycle can replenish it so pyruvate is reduced to lactate
- 2 NAD+ are produced from the 2 pyruvate from glycolysis, which is exactly enough to keep glycolysis occurring
- pyruvate is converted to lactate using lactate dehydrogenase
what is ethanol fermentation
- yeast makes ethanol from pyruvate, the human liver enzyme oxidizes ethanol to acetaldehyde
- TPP is thiamine pyrophosphate, a cofactor derived from Vitamin B1 and involved when C-C bond is broken
What is the Warburg effect?
- tumor cells initially lack the capillary network to supply suffienicent oxygen
- tumor cells adapt by dramatically increasing glucose uptake and glycolysis (10X faster than non-cancerous cells)
- occurs primarily by increasing the synthesis of hexokinase and glucose transporter
- drug inhibitors that inhabit hexokinase have been devloped as anticancer treatment
advantages of glucose with PET
- glucose can be traced using position emission tomography, PET, in patients given fdG
- high uptake always seen in the brain and bladder
What is the fate of each pyruvate under aerobic conditions
- when oxygen is available, oxidation occurs and pyruvate is produced in the cytosol enters the mitochondria
- first pyruvate (3-C) --> acetyl-CoA (2-C) + CO2 via pyruvate dehydrogenase complex from which the acetyl-CoA enters the citric acid cycle
- pyruvate dehydrogenase complex is a massive multi-enzyme complex attached to the inner side of the inner mitochondrial membrane (matrix)
What are the layers of the mitochondria
- outer membrane
- inner membrane
- matrix
what is the outer membrane of the mitochondria
- freely permeable to small molecules and ions
what is the inner membrane of the mitochondria
- impermeable to most small molecules and ions, including H+
contain:
- respiratory electron carriers
- ADP-ATP translocase
- ATP synthesis
- outer membrane transporters
what is the matrix of the mitochondria
- contains pyruvate dehydrogenase, citric acid cycle enzymes, fatty acid beta-oxidation enzymes, amino acid oxidation enzymes, DNA and ribosomes, and more
Pyruvate to Acetyl CoA
- used pyruvate dehydrogenase complex
- irreversible and important control point linking glycolysis and and TCA cycle
- inhibited by ATP, acetyl-coA, fatty acids, and NADH, aka high energy '
- activated by AMP, CoA, NAD+, aka low energy
What is the citric acid cycle
- the action of pyruvate dehydrogenase produces more NADH, committing the cell to anerobic metabolism (need oxygen)
- completes the oxidation of C2 (Aceytl-coA) in mitochondria
- acetate is oxidized to CO2 and H2O
- 8 step
Step 1 of Citric Acid Cycle
- Acetyl-CoA + Oxaloacetate --> Citrate
- Uses citrate synthase enzyme
- exergonic due to highly exergonic free energy change accompanying hydrolysis of acetyl-CoA
Step 2 of Citric Acid Cycle
- Citrate <--> Isocitrate
- Uses aconitase enzyme
- two reactions catalyzed by aconitase results in isomerization of citric to isocitrate
- non-favourable and pulled forward by removal of product in the next step
Step 3 of Citric Acid Cycle
- Isocitrate --> α-ketoglutarate
- Uses isocitrate dehydrogenase
- oxidative decarboxylation that uses NAD+ or NADH as electron acceptors
- favourable reaction
Step 4 of Citric Acid Cycle
- α-ketoglutarate --> succinyl-CoA
- Uses α-ketoglutarate dehydrogenase complex that is inhibited by NADH and succinyl-CoA
- CoA + NAD+ --> NADH + CO2
- irreversible and oxidative decarboxylation
- identical mechanism to pyruvate dehydrogenase
Step 5 of Citric Acid Cycle
- Succinyl-CoA <--> Succinate
- Uses succinyl-CoA synthetase enzyme
- GDP + Pi <--> GTP + CoA
- free energy related when high energy thioester is hydrolysed and conversed in the formation of GTP and GDP and Pi
- substrate level phosphorylation
Step 6 of Citric Acid Cycle
- Succinate to fumarate via succinate deH2ase
- Succinate-CoA synthetase reaction mechanism
- succinate is oxidized to fumarate and free energy is stored in reduced FAD which is convantly attached to the enzyme
- located in mitochondria membrane: membrane bound enzyme and all other citric acid enzymes are soluble
Step 7 of Citric Acid Cycle
- Fumarate <--> L-Malate
- Uses fumarase enzyme
- fumerase stereospecifically adds water across c=c
- maleate and D-malate are not substrates for this enzyme
Step 8 of Citric Acid Cycle
- L-Malate <--> Oxaloacetate
- Uses malate dehydrogenase enzyme
- NAD+ <--> NADH
- endergonic with equilibrium to far left
- OAA is used up very fast in a new round of the cycle in the highly exergonic citrate synthesis step
- by coupling, the overall reaction become -2.5 favouring the citric acid cycle
citric acid cycle summary
- all C atoms in glucose have been converted to CO2
- 4 steps involved oxidations that conserved energy by reducing the electron carriers (3 NADH + FADH2) plus 1 high energy phosphate formed (GTP)
- 6-8 regenerated OAA so there is no net consumption or production of intermediates
- functions as catalysis
What is the electron transport chain?
- series of proteins with prosthetic groups that become oxidized and reduced alternatively as electrons (and H atoms in some steps) are passed along
- electrons entering the ETC are energy rich and as they passed down, they loss free energy. The energy is conserved by ATP formation involving multi-enzyme complex, ATP synthase
- process of ATP formation is called oxidative phosphorylation and coupled to ETC
What are the electron carriers involved in the ETC
- Coenzyme Q = Ubiquinone
- Cytopchromes
- Fe-S proteins
Coenzyme Q = Ubiquinone
- has long lipid double aliphatic tail and can diffuse through the membrane accepting and donating e-
- picks up 2e-
Cytopchromes
- all but cytochrome c are integral proteins
- they have different amino acid sequences and bind slightly different iron-containing hemes
- cytochrome c is a peripheral membrane protein that binds Heme C covanlently via Cys residues
- the standard potential, a measure of freedom energy of the electron is different in each protein
Fe-S proteins
- free energy of the bound and related electron is different in different proteins
- can be Fe2+ or Fe3+
Arrangement of electron carriers in ETC
- Carriers are arranged in a way that the given carrier can accept electrons from one carrier only and pass them on to one carrier only
- carriers with high negative reduction potential (E') pass electrons to carriers with high positive reduction potentials
Complex 1 (NADH dehydrogenase)
- Consists of more than 25 proteins, 7 Fe-S centres and FMN
- UQH2 diffuses through the membrane to complex 3
- 4H+ are pumped across the membrane
Complex 2 (succinate dehydrogenase)
- Consists of 4 proteins including Fe-S proteins and convalently bound FAD
- only membrane bound enzyme of ETC
- QUH2 diffuses to complex 3
complex 3 (UQ-Cty c Oxidoreductase)
- 10 proteins containing cytochrome and Fe-S protein
- the electron transfer path is called Q-cycle
- last electron acceptor is cytochrome c which dissociates from complex 3 and carrier one electron as a time to complex 4
- 4H+ are pumped across the membrane by complex 3
Complex 4 (cytochrome c oxidase)
- consist of 10 proteins including cytochrome a and a3
- 2 e- passes from 2 cytochrome c from complex 4 to 1/2O2
- the 2 e- come from 1 NADH
- 2H+ are pumped across the membrane for each 2 electrons transferred through complex 4
what is the energetics of an electron transfer
- energy released in discrete steps during electron transport and stored in the form of a proton concentration gradient
- electrons in NADH have high energy and electrons in water have low energy creating this down hill electron flow
- the electron flow makes enough energy to make ATP from ADP + Pi forming about 2.5 ATP per NADH turned into 1/2O2
- succinate oxidation yields -150 kJ/mol vs -220 so it only forms 1.5 ATP per NADH to 1/2O2
how is free energy in an electron flow converted into ATP
- During the transport of 2e- down the ETC from NADH, complex 1, 2, 3, 4 transport 10H+ from the matrix to the intermitochondrial space doing electrochemical work using the free energy released during electron flow
- chemical/ pH gradient is built top across the membrane with OH- in the matrix and H+ in the space
- an electrochemical gradient is built up across inner membrane as positive and negative charges are seperated. The matrix becomes negative and the IMS positive
What does using NADH in ETC chain yield
- 10H+ pumped and 1/2O2
- -220kJ/mol
- 2.5 ATP
What does using FADH2 (succinate) in ETC chain yield
- 6H+ pumped and 1/2O2
- -150 kJ/mol
- 1.5 ATP
how is the energy converted to ATP
- once the 4H+ are returned into the martrix, enough energy is provided to make 1 ATP
What is F0 F1 ATP synthesis
- F1 is a peripheral membrane protein with subunits, alpha, beta, and gama
- F0 is a tetrameric integral membrane protein H+ channel
- H+ flows through F0 forming torque that is transmitted to F1 via long arm, that drives the opposite rotation of F0 and F1
- rotation energy is used to release ATP from binding site
- electron transport and phosphorylation are tightly coupled so inhibition of either shuts down the process
- oxidative phosphorylation is regulated by ADP an d phosphate
- enzyme ATP/ADP translocase moves ATP into the cytoplasm and ADP into mitochondria
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