Carbohydrates Main Metabolic Pathways

Glycolysis ‘sweet’ & ‘splitting’

an amphibolic pathway →function in both anabolic & catabolic processes

occurs in the __________

simple sugar (i.e. glucose) split into pyruvate

can proceed under anaerobic conditions

all living organisms used this process

divided into two phases –energy investment (phase I) & energy generating (phase II)

Phase I: Energy Investment

Phase II: Energy Generating

Reaction ATP/glucoseGlucose G-6-P -1

F-6-P FBP -1

2 1,3-BPG 2 3-PG +2

2 PEP 2 pyruvate +2Net production +2

Use and production of ATP in glycolysis:

glycolytic net reaction:

Glucose + 2 ADP + 2 PO42- + 2 NAD+

2 pyruvate + 2 NADH + 2 H2O + 2 ATP

10 enzymes

further metabolized

Regulation of Glycolysis

by allosteric regulation of these enzymes:

Enzyme Activator InhibitorHexokinase Glucose-6-

phosphate, ATPPhosphofructokinase-1

(PFK-1)Fructose-2,6-bisphosphate,

AMP

Citrate, ATP

Pyruvate kinase Fructose-1,6-bisphosphate,

AMP

Acetyl-CoA, ATP

Fates of Pyruvate: anaerobically or aerobically???

aerobically by converting into acetyl-CoA & into citric acid cycle

anaerobically by fermentation

Fermentation does not release all the available energy in a

molecule (i.e. products are not fully oxidized), it only allows glycolysis to continue produce only 2 ATP per glucose consumed

for glycolysis to operate anaerobically, NADH must be reoxidized to NAD+ by transferring its e- to an e- acceptor (i.e. pyruvate)

________ & ethanol fermentations

Lactate Fermentation

produced by the muscles when O2 is in short supply, also occurs in some bacteria and some fungi

can lead to O2 debt in muscles after strenuous exercise needs additional O2 to oxidize lactate

lactate dehydrogenase

carried out by yeast and some types of bacteria important in bread-making, brewing, and

wine-making

Ethanol Fermentation

pyruvate decarboxylase

alcohol dehydrogenase

Entry of other carbohydrates into glycolysis

polysaccharides → starch & glycogen hydrolyzed to glucose by amylase

disaccharides → hydrolyzed to different monosaccharide pairs

monosaccharides: fructose 2 different pathways

galactosemannose

Fructose Metabolism

METABOLISM

GALACTOSE

Glycolysis

METABOLISM

MANNOSE

Glycolysis

Fates of Pyruvate: Aerobic Metabolism

mechanism whereby energy of the chemical bonds in food/glucose is stored & used to drive ATP synthesis

occurs in __________ processes involved:

citric acid cycle e- transport chain (ETC)oxidative phosphorylation

Citric Acid Cycle also known as Kreb’s cycle or tricarboxylic acid

(TCA) cycle

converts pyruvate to CO2, NADH, FADH2, GTP

amphibolic: catabolic acetyl-coA oxidized to form CO2 &

energy is conserved anabolic producing precursors for biosynthetic

pathways

acetyl CoA from fatty acid breakdown and amino acid degradation products are also oxidized

starts after pyruvate (from glycolysis) is first converted into acetyl-CoA in a pyruvate decarboxylation reaction:

Pyruvate dehydrogenase complex

Pyruvate Dehydrogenase Complex

Enzyme Activity Function CoenzymePyruvate

dehydrogenase(E1)

Pyruvatedecarboxylation

TPP

Dihydrolipoyl transacetylase

(E2)

Transfer acyl grp. to lipoic acid

Lipoic acid, CoA

Dihydrolipoyl dehydrogenase

(E3)

Reoxidation of dihydrolipoamide

NAD+, FAD

Action of Pyruvate Dehydrogenase Complex

TPP of E1 reacts with pyruvate, which undergoes decarboxylation (i.e. loss of CO2). The acetyl portion becomes a hydroxyethyl derivative covalently attached to TPP (HETPP).

In the next several steps, HETPP is converted to acetyl-coA by E2. Lipoic acid of E2 reacts with HETPP forming acetylated lipoic acid and free TPP.

Acetyl group then transferred to sulfhydryl grp of CoA.

Reduced lipoic acid is reoxidized by E3. FADH2 is reoxidized by NAD+ to form FAD required for oxidation of next reduced lipoic acid residue.

1

2

3

4

The 8 steps of citric acid cycle involving:

1. condensation2. isomerization3. oxidation4. oxidation5. ____________6. oxidation7. hydration8. oxidation

1. 2.

3.

4.5.

6.

7.

8.

overall reaction of citric acid cycle:

for each turn of cycle, ________ ATP molecules are produced: one directly from the cycle (GTP)

11 from the re-oxidation of the three NADH and one FADH2 molecules produced by the cycle by oxidative phosphorylation

Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O

CoA + 2 CO2 + 3 NADH + FADH2 + GTP + 3 H+

Overall, the cycle speeds up when cellular energy

levelsare low (high ADP, low ATP and NADH)

and slows down as ATP (and then

NADH2,succinyl CoA and

citrate)accumulates.

Citric acid cycle regulation

anabolic processes drain citric acid cycle of molecules required to sustain its role in energy production need to replenish intermediates

reactions that form intermediates of the cycle are called anaplerotic reactions

they serve to replenish TCA cycle how? replenishing oxaloacetate by PEP carboxylase &

pyruvate carboxylase produce malate from pyruvate by malic enzyme transamination reactions of amino acids by

transaminases in plants, some bacteria & algae employ

__________ cycle

Anaplerotic Reactions

Citric acid cycle as source of anabolic

processes

Anaplerotic reactions of citric acid cycle

modified version of citric acid cycle used by plants, some fungi, algae, protozoans and bacteria bypass steps where C is lost as CO2 in citric acid

acycle conserve 4C compounds for biosynthesis

occurs in glyoxysomes (in plants) or cytoplasm (other eukaryotic organisms & bacteria)

allows organisms to use fats for the synthesis of carbohydrates, a task which vertebrates, including humans, cannot perform

acetyl-CoA used is derived from breakdown of fatty acids allows for gluconeogenesis from fatty acids (impossible in animals)

net result is production of glucose from fatty acids

Glyoxylate Cycle

Glyoxylate Cycle

Electron Transport Chain (ETC)

couples a reaction between an electron donor (i.e. NADH) and an electron acceptor (i.e. O2) to the transfer of H+ ions across a membrane, through a set of mediating biochemical reactions

generate the majority of ATP, the main energy intermediate in living organisms

used for extracting energy from sunlight (photosynthesis) and from redox reactions such as the oxidation of sugars (respiration)

consists of a spatially separated series of redox reactions in which e- are transferred from a donor molecule to an acceptor molecule drived by free energy

components located in inner mitochondrial membrane organized into 4 complexes:

complex I NADH dehydrogenase complex II succinate dehydrogenase complex III _____________________ complex IV cytochrome oxidase

* e- transporters in ETC according to

potential in giving off e-

ETC components

complex I – NADH dehydrogenase transfer e- from NADH to

ubiquinone/CoQ consists of 1 FMN, 7 Fe-S

centers

complex II – succinatedehydrogenase transfer e- from succinate

to CoQ consists of 2 Fe-S centers,

FAD)

complex III – cytochromebc1 transfer e- from reduced

CoQ to cytochrome c 2 cytochrome b, 1

cytochrome c1, 1 Fe-S center

complex IV – cytochromeoxidase reduction of 4e- of O2 to

H2O cytochrome a, cytochrome

a3, 2 Cu (A & B)

Oxidative Phosphorylation ATP synthesis linked to the

oxidation of NADH and FADH2 by e- transport through the respiratory chain(energy produced by ETC is stored in phosphorylation of ADP to ATP)

chemiosmotic coupling theory (Mitchell, 1961):

when e- passed thru ETC, H+

is transferred from the matrix to intermembrane space

H+ electrochemical gradient/proton motive force H+ reenter into matrix through ATP ___________

ATP synthesis

Structure of ATP synthase

Site for ADP + Pi attachment to

synthesize ATP

Channel H+ & anchor structure

to inner membrane

Matrix

Inner membrane

Coupling and Respiratory Control

e- transport is tightly coupled to ATP synthesis: e- do not flow through ETC to O2 unless ADP is simultaneously phosphorylated to ATP

therefore, ADP↑, ETC proceeds; ADP↓, ETC slows down

Reoxidation of Cytosolic NADH

cytosolic NADH produced from glycolysis in cytosol cannot cross the inner mitochondrial membrane and enter mitochondria to be reoxidized

reoxidized through:glycerol phosphate shuttle: produced 2

ATPmalate aspartate shuttle: produced 3 ATP

Glycerol Phosphate Shuttle

1

2

-ketoglutarate-malate transporter

Aspartate-glutamate transporter

Malate-Aspartate Shuttle

Glucose

Pyruvate

Acetyl-CoA

TCA CYCLE

GLYCOLYSIS

Glycerol Phosphate Shuttle

Malate-AspartateShuttle

2 2

2 NADH

2 NADH 4 6

6 6

2 2

6 NADH 18 18

2 FADH2 4 4

Net yield 36 38

Yield of ATP from glucose oxidation

Photosynthesis a process that converts carbon

dioxide into organic compounds, especially sugars, using the energy from sunlight

occurs in photoautotrophs overall equation of photosynthesis:

begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls held inside organelles called chloroplasts (in plants and algae) or plasma membrane (in bacteria)

2 main stages: light-dependent reaction light-independent reaction

The site of photosynthesis

chlorophyll is the main photosynthetic pigment where light energy is trapped and turned into chemical energy

the main colour of light absorbed by chlorophyll is red and blue; the main colour reflected (not absorbed) is green

other pigments involve as accessory pigments carotenoids (i.e. xanthophylls, carotenes) and phycobilins (i.e. phycoerythrin and phycocyanin)

Structure of

chlorophyll

The light-dependent reaction of photosynthesis

a process whereby light energy is converted into chemical energy (i.e. ATP and NADPH)

takes place on the thylakoid membrane

involves four major protein complexes: photosystem I (PSI), photosystem II (PSII), cytochrome b6f complex and ATP synthase

chlorophyll and other accessory (or antenna) pigments in light-harvesting complex (a part of a photosystem) absorb light (or photons) and funnel the energy to reaction center chlorophyll a

energy is transferred in a form of excited electrons (e-) resonance energy transfer

in PSI, the reaction center chlorophyll a is called P700 and in PSII it’s P680

the pathway of e- transfer through e- transport chain (ETC) of e- carriers the Z-scheme

products of light-dependent reactions: 6 O2 through splitting of 12 H2O by H2O-splitting complex or

oxygen-evolving complex in PSII 12 ATP through the process of photophosphorylation 18 NADPH through the catalytic activity of the enzyme

ferredoxin NADP+ oxidoreductase that reduces NADP+ to NADPH which accepts e- from feredoxin (Fdx)

O2 released to enviroment via stroma; ATP and NADPH used for light-independent reaction

photon

photon

The Z-scheme of electron flow in photosynthesis

Photophosphorylation

production of ATP using the energy of sunlight made by an enzyme called ATP synthase powered by a transmembrane electrochemical

potential gradient, usually in the form of a proton gradient produced by ETC chemiosmosis and proton motive force

2 types: non-cyclic photophosphorylation

cyclic photophosphorylation

cyclic photophosphorylation: no NADPH is produced, only ATP

occurs when cells may require additional ATP or when there is no NADP+ to reduce to NADPH

Light-independent reaction of photosynthesis chemical reactions that convert carbon dioxide

and other compounds into sugars (i.e. glucose) also known as dark reaction or Calvin cycle occurs in stroma of chloroplast requires ATP and NADPH from light-dependent

reaction 3 phases:

carbon fixation reduction reactions

regeneration of co2 acceptor

the process by which RuBP has O2 added to it by the enzyme Rubisco, instead of CO2

tends to occur when there is a high concentration of O2 relative to CO2

produces no ATP (energy for cells) and leads to a net loss of carbon and nitrogen (as ammonia), slowing the growth of plants

occurs in 3 different organelles: chloroplast, peroxisome and mitochondria

Photorespiration

Fate of glucose made from photosynthesis

Glycogen Metabolism

glycogen is stored in granules in muscles or liver cells

enables the blood glucose level to be maintained between meals, and also provides an energy reserve for muscular activity

a process whereby glycogen is either catabolized/degraded (glycogenolysis) or anabolized/synthesized (glycogenesis)

connections to other pathways: glycogen is broken down to and can be made from glucose-

1-P glucose-1-P to glucose-6-P (phosphoglucomutase reversible

reaction) glucose-6-P to glucose (liver and kidney only ---> for

bloodstream) glucose-6-P from glucose via kinase (hexokinase or

glucokinase) glucose-6-P to and from glycolysis (catabolism) and

gluconeogenesis (anabolism) glucose-6-P to pentose phosphate pathway (not reversible) for reducing equivalents (NADPH) and ribose for nucleic acids

Glycogenolysis conversion of glycogen polymers to glucose monomers takes place in the muscle and liver tissues control by hormones epinephrine and/or glucagon involved 3 enzymes: glycogen phosphorylase glycogen phosphorylation to

glucose-1-P glycogen debranching enzyme transfers glycogen branch; 2 activities: glucanotransferase and glucosidase phosphoglucomutase converts _______________ to

glucose-6-P glucose-6-P (i.e. product) then used in glycolysis (in

muscle) or convert to glucose (in liver or kidney)

Phosphorylase can cleave (14) linkages only to within 4 residues of an (16) branch point (i.e. limit branch)

Action of glycogen

debranching enzyme

removes the α(16) branches;

so allows phosphorylase to

continue degrading glycogen molecule

Action of phosphoglucomutase

[glucose-1,6-bisphosphate]

Glycogenesis a process of glycogen _______________ activated during rest periods following the Cori cycle

(in liver), and also activated by insulin in response to high glucose levels

glucose from diet converted into glucose-6-P by hexokinase

involved 4 enzymes for glycogen synthesis from glucose-6-P: phosphoglucomutase converts glucose-6-P to

glucose-1-P UDP-glucose pyrophosphorylase

glycogen synthase amilo-(1,41,6)-transglycosylase a glycogen

branching enzyme

Conversion of glucose monomer into glycogen

polymer

Action of phosphoglucomutase in glycogenesis

Action of UDP-glucose pyrophosphorylase

Action of glycogen synthase

needs pre-existing glycogen primer attached to ______________ protein

Formation of glycogen primer glycogenin enzyme acts as primer, to which further glucose

monomers may be added achieved by catalyzing the addition of glucose to itself by first

binding glucose from UDP-glucose to the hydroxyl group of Tyr-194

glycogenin then catalyzes glucosylation at C4 of the attached glucose, with UDP-glucose again being the glucose donor, process repeated until a short linear polymer of glucose with (14) is built up

glycogen synthase, can only add to an existing chain of at least 8 glucose residues; once sufficient residues have been added, glycogen synthase takes over extending the chain

glycogenin remains covalently attached to the reducing end of the glycogen molecule

Glycogen branching process

Branches are important because enzymes that degrade or synthesize glycogen work only at the ends of glycogen molecule.

Existence of many termini allows a far more rapid rate of synthesis and degradation than would be possible with a nonbranched polymer.

Regulation of glycogen metabolism

both glycogen synthesis and degradation are tightly controlled via:

allosteric regulation, and,covalent modification

of glycogen synthase and phosphorylase

covalent modification is under close hormonal control (i.e. glucagon/epinephrine and insulin)

allosteric regulation by: glucose-6-P activates ___________ (synthesis) and inactivates phosphorylase (

degradation and synthesis) ATP activates synthase (synthesis) and inactivates phosphorylase ( degradation and

synthesis) glucose inactivates ________________ (

degradation and synthesis)

HORMONAL

EFFECTS

ON

GLYCOGEN

METABOLISM

turn glycogen synthesis off, and mobilizes glycogen stores by turning degradation on

activate adenylatecyclase and cAMPcascade: phosphorylationinactivates glycogen

synthase and activates glycogen phosphorylase

How epinephrine or glucagon works on

glycogen metabolism?

synthesis of glucose from non-carbohydrate precursors pyruvate, lactate, glycerol, amino acid, acetate, propionate

important for the maintenance of blood glucose levels during starvation or during vigorous exercise

occurs in liver a ‘reverse glycolysis’ process EXCEPT: 7 out of 10 glycolytic steps are reversible, 3

steps are not alternate reactions required enzymes not involved: pyruvate kinase,

phosphofructokinase and hexokinase

Gluconeogenesis

Phosphoenolpyruvate (PEP) pyruvate by pyruvate

kinase

fructose-6-P fructose-1,6-bisP by phosphofructokinase-

1

glucose glucose-6-P by hexokinase

Glycolysis

Gluconeogenesis

2 pyruvate + 2 NADH + 4 ATP + 2 GTP + 2 H+

Glucose + 2 NAD+ + 4 ADP + 2 GDP + 6Pi

overall reaction of gluconeogenesis (pyruvate as precursor):

Other precursors of gluconeogenesis

glycerolglycerol glycerol-3-P

DHAP

lactate converted into pyruvate

by lactate dehydrogenase in liver cells Cori Cycle

amino acids C skeleton catabolised

into pyruvate/TCA Cycle intermediates

pyruvate (peripheral tissues) Ala pyruvate (liver): the Glucose-Alanine Cycle

aspartate (from Urea Cycle) fumarate malate ___________

Glucose-Alanine Cycle

also serves other purposes: recycles carbon skeletons between muscle and liver

transports NH4+ to the liver and is converted into ________

Regulation of gluconeogenesisGlucose

Fructose-6-P

Fructose-1,6-bisP

PEP

Pyruvate

Lactate

GluconeogenesisGlycolysis

phosphofructokinase fructose-1,6-bisphosphatase

pyruvate kinase

PEP carboxykinase

pyruvate carboxylase

AMP

Fructose-2,6-bisphosphate

acetyl-CoA(+)

(-)

(-)

Oxaloacetate

AMP(+)

alternative to glycolysis

also known as hexose monophosphate shunt or phosphogluconate pathway

occurs in tissues involved in lipid biosynthesis liver, mammary gland, adipose tissue, adrenal cortex

core set of reactions: oxidize glucose 6-P to ribose 5-P and generate NADPH

main functions: produced NADPH for

cellular biosynthesis produced ribose-5-P for

nucleic acid synthesis metabolized pentose sugar

from nucleic acid rearrange carbohydrates C

skeleton as glycolytic/gluconeogenic

intermediates

2 phases: oxidative & non-oxidative

Pentose-Phosphate Pathway

convert glucose 6-P into ribulose 5-P, generating 2 NADPH molecules

isomerization of ribulose 5-P to ribose 5-P linkage of the pentose phosphate pathway to glycolysis via

transketolase and transaldolase

Ribulose-5-phosphate epimerase Ribulose-5-phosphate

isomerase

rearrangement of the C skeleton (non-oxidative stage):

(6) C5 + C5 C7 + C3

(7) C7 + C3 C6 + C4

(8) C5 + C4 C6 + C3

(sum) 3 C5 2 C6 + C3

enzymes involved: transketolase transfer 2C transaldolase transfer 3C

C6 (F-6-P) is recycled to produce more NADPH or goes into glycolysis

C3 (GAP) enter glycolysis/gluconeogenesis

Regulation of pentose-phosphate pathway

depends on conditions/cell needs: when cell needs NADPH but not ribose 5-P, ribose 5-P is converted to glycolytic intermediates and enter glycolysis

when cell needs ribose 5-P, fructose 6-P & glyceraldehyde 3-P taken from glycolysis & converted to

ribose 5-P

enzyme G-6-P dehydrogenase activity: rate limiting & irreversible NADPH as allosteric inhibitor [NADPH], activity

also inhibited by fatty acid CoA acyl ester

Alternative pentose-phosphate pathway: to meet varying metabolic

needs

The End!