Biomedical Education - Metabolism - TCA cycle

© 2003 By Default!Dr Fiona Coulson– Biomedical Sciences, CQU BMED19003 Lecture 3&4 – Metabolism

Clinical BiochemistryClinical Biochemistry

BMED19003BMED19003

Lecture 3 and 4 – TCA cycle, electron Lecture 3 and 4 – TCA cycle, electron transport chain and beta-oxidationtransport chain and beta-oxidation

Dr Fiona CoulsonDr Fiona Coulson


More metabolism More metabolism –– learning objectives learning objectives

• follow the sequence of carbon molecules from glycolysis to TCA cycle and onto electron transport

• complete the full cycle of ATP production from glycolysis in the presence of oxygen

• understand the importance of NADH and FADH2

• contrast fat breakdown with that of glycolysis


EnergyEnergy

Cells need energy to workCells need energy to work

This energy comes from the food we eatThis energy comes from the food we eat

Food is broken down into its component Food is broken down into its component molecules and is absorbed into the portal molecules and is absorbed into the portal bloodstream or lymphaticsbloodstream or lymphatics

What happens after that?What happens after that?


What next?What next?

The next step is to either store the food or The next step is to either store the food or use ituse it

We have started talking on how we We have started talking on how we recover energy from carbohydrates recover energy from carbohydrates through the complete oxidation of glucosethrough the complete oxidation of glucose


Carbohydrate metabolismCarbohydrate metabolism


Krebs cycle – preparatory stepKrebs cycle – preparatory step

Occurs in the mitochondrial matrix and is Occurs in the mitochondrial matrix and is fueled by pyruvic acid and fatty acidsfueled by pyruvic acid and fatty acids

Pyruvic acid is converted to acetyl CoA in three Pyruvic acid is converted to acetyl CoA in three main steps:main steps:– DecarboxylationDecarboxylation

• Carbon is removed from pyruvic acidCarbon is removed from pyruvic acid

• Carbon dioxideCarbon dioxide is releasedis released

– OxidationOxidation• Hydrogen atoms are removed from pyruvic acidHydrogen atoms are removed from pyruvic acid• NADNAD+ + is reduced to NADH + His reduced to NADH + H++

• Formation of acetyl CoA – the resulting acetic acid is Formation of acetyl CoA – the resulting acetic acid is combined with coenzyme A, a sulfur-containing coenzyme, combined with coenzyme A, a sulfur-containing coenzyme, to form acetyl CoAto form acetyl CoA


An eight-step cycle in which each acetic An eight-step cycle in which each acetic acid is decarboxylated and oxidized, acid is decarboxylated and oxidized, generating: generating: – Three molecules of NADH + HThree molecules of NADH + H+ + = 9 ATP= 9 ATP– One molecule of FADHOne molecule of FADH2 2 = 2 ATP= 2 ATP– Two molecules of COTwo molecules of CO22

– One molecule of ATPOne molecule of ATP– total potential of 12 ATPtotal potential of 12 ATP

For each molecule of glucose entering For each molecule of glucose entering glycolysis, two molecules of acetyl CoA glycolysis, two molecules of acetyl CoA enter the Krebs cycleenter the Krebs cycle

Krebs cycleKrebs cycle


Electron transport chain (ETC)Electron transport chain (ETC)

Food (glucose) is oxidized and the released Food (glucose) is oxidized and the released hydrogens:hydrogens:– Are transported by coenzymes NADH and Are transported by coenzymes NADH and

FADHFADH22

– Enter a chain of proteins bound to metal atoms Enter a chain of proteins bound to metal atoms (cofactors) (cofactors)

– Combine with molecular oxygen to form waterCombine with molecular oxygen to form water– Release energyRelease energy

The energy released is harnessed to attach The energy released is harnessed to attach inorganic phosphate groups (Pinorganic phosphate groups (Pii) to ADP, ) to ADP, making ATP by oxidative phosphorylationmaking ATP by oxidative phosphorylation


Mechanisms of oxidative phosphorylationMechanisms of oxidative phosphorylation

The hydrogens delivered to the chain are The hydrogens delivered to the chain are split into protons (Hsplit into protons (H++) and electrons) and electrons

– The protons are pumped across the inner The protons are pumped across the inner mitochondrial membrane by:mitochondrial membrane by:• NADH dehydrogenase (FMN, Fe-S)NADH dehydrogenase (FMN, Fe-S)

• Cytochrome b-cCytochrome b-c11

• Cytochrome oxidase (a-aCytochrome oxidase (a-a33))

– The electrons are shuttled from one acceptor The electrons are shuttled from one acceptor to the nextto the next


Electrons are delivered to oxygen, Electrons are delivered to oxygen, forming oxygen ionsforming oxygen ions

Oxygen ions attract HOxygen ions attract H++ to form water to form water

HH++ pumped to the intermembrane space: pumped to the intermembrane space:– Diffuses back to the matrix via ATP synthase Diffuses back to the matrix via ATP synthase – Releases energy to make ATPReleases energy to make ATP

Mechanisms of oxidative phosphorylationMechanisms of oxidative phosphorylation


Electric energy gradientElectric energy gradient

The transfer of energy from NADH + HThe transfer of energy from NADH + H++ and FADH and FADH22 to to oxygen releases large amounts of energyoxygen releases large amounts of energy

This energy is released in a stepwise manner through the This energy is released in a stepwise manner through the electron transport chainelectron transport chain

The electrochemical proton gradient across the inner The electrochemical proton gradient across the inner membrane:membrane:– Creates a pH gradient Creates a pH gradient – Generates a voltage gradientGenerates a voltage gradient

These gradients cause HThese gradients cause H++ to flow back into the matrix via to flow back into the matrix via ATP synthaseATP synthase


ATP synthaseATP synthase

The enzyme consists of three parts: a rotor, a The enzyme consists of three parts: a rotor, a knob, and a rodknob, and a rod

Current created by HCurrent created by H++ causes the rotor and rod causes the rotor and rod to rotateto rotate

This rotation activates catalytic sites in the This rotation activates catalytic sites in the knob where ADP and Pknob where ADP and Pii are combined to make are combined to make

ATPATP


Summary of ATP productionSummary of ATP production


Fate of lipids - vertebrates

Lipoproteins


Figure 23.1Scanning electron micrograph of an adlpose cell (fat cell). Globules of triacylglycerols occupy most of the volume of such cells.


Lipid metabolismLipid metabolism Strong links with CHO metabolism through the formation of Strong links with CHO metabolism through the formation of

TCA intermediatesTCA intermediates

Oxidation of lipids provides about 50% of the energy needs Oxidation of lipids provides about 50% of the energy needs of the liver, kidney, heart and skeletal muscle of the liver, kidney, heart and skeletal muscle

Mobilised in response to prolonged exercise, starvation or Mobilised in response to prolonged exercise, starvation or fearfear

Lipases activated within adipocytesLipases activated within adipocytes

Free fatty acids enter bloodstream and combine with Free fatty acids enter bloodstream and combine with albumin albumin


Lipid metabolismLipid metabolism


OxidationOxidation of long-chain fatty acids yields energy of long-chain fatty acids yields energy

In mammals, up to ~80% of energy requirements are met In mammals, up to ~80% of energy requirements are met by fatty acid oxidationby fatty acid oxidation

Acetyl-CoA Acetyl-CoA also produced from fatty acids→ oxidised to also produced from fatty acids→ oxidised to COCO22 in the citric acid cycle in the citric acid cycle

Acetyl-CoA Acetyl-CoA important molecule associated with lipid important molecule associated with lipid metabolismmetabolism

Electrons removed from fatty acids (oxidation) are Electrons removed from fatty acids (oxidation) are transferred to the electron transport chain → drives transferred to the electron transport chain → drives synthesis of synthesis of ATPATP

Overview of fatty acid oxidation


ββ-oxidation:-oxidation: 4-step process resulting in the 4-step process resulting in the production of C2 acetyl fragment from fatty production of C2 acetyl fragment from fatty acidacid

complete oxidation of fatty acids yields complete oxidation of fatty acids yields COCO22 and H and H22OO


-Oxidation of Fatty Acids-Oxidation of Fatty Acids

Repeated sequence of 4 reactions Repeated sequence of 4 reactions

Strategy: 3 reactions to create a carbonyl group Strategy: 3 reactions to create a carbonyl group (C=O)on (C=O)on -C -C

Then fourth reaction cleaves the "Then fourth reaction cleaves the "-keto ester" -keto ester"

Products: (i) Products: (i) acetylacetyl-CoA and (ii) fatty acid two -CoA and (ii) fatty acid two carbons shorter carbons shorter


Figure 23.6Fatty acids are degraded by repeated cycles of oxidation at the -carbon and cleavage of the CC bond to yield acetate units.


Acyl-carnitine/carnitine transporter

Fatty acid catabolism occurs inside the mitochondrion – in matrix

Fatty acyl CoA cannot cross the inner membrane, so fatty acyl group is transferred to carnitine (carnitine can cross the membrane)

Assisted by specific carnitine carrier protein


Summary of Summary of -Oxidation-Oxidation

Repetition of the cycle yields a succession of Repetition of the cycle yields a succession of acetateacetate units units

Thus, palmitic acid (C16) yields eight acetyl-Thus, palmitic acid (C16) yields eight acetyl-CoAs (to TCA cycle – oxidation to COCoAs (to TCA cycle – oxidation to CO22 & H & H22O)O)

Complete Complete -oxidation of one palmitic acid yields -oxidation of one palmitic acid yields 106 molecules of ATP 106 molecules of ATP

Large energy yield is consequence of the Large energy yield is consequence of the highly highly reduced state of fatty acids reduced state of fatty acids (….[CH2](….[CH2]nn….)….)

This makes fatty acids the fuel of choice for This makes fatty acids the fuel of choice for migratory birds, animals (hibernation/famine/etc)migratory birds, animals (hibernation/famine/etc)

Biomedical Education - Metabolism - TCA cycle

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