Carbohydrate metabolism

CARBOHYDRATE METABOLISM

Chapter 3A. Coupled reactionsThe additivity of free energy changes

allows an endergonic reaction to be driven by an exergonic reaction under the proper conditions. (thermodynamic basis for the operation of the metabolic pathways since most of these reaction sequences comprise endergonic as well as exergonic reactions.

BREAK-DOWN OF GLUCOSE TO GENERATE ENERGY

- Also known as Respiration. - Comprises of these different

processes depending on type of organism:

I. Anaerobic Respiration II. Aerobic Respiration

ANAEROBIC RESPIRATION

Comprises of these stages: glycolysis: glucose 2 pyruvate + NADH fermentation: pyruvate lactic acid or ethanol cellular respiration:

AEROBIC RESPIRATION

Comprises of these stages: Oxidative decarboxylation of pyruvate Citric Acid cycle Oxidative phosphorylation/ Electron Transport

Chain(ETC)

STARCHY FOOD

α – AMYLASE ; MALTASES

Glycolysis in cytosol

Brief overview of catabolism of glucose to generate energy

Glucose converted to glu-6-PO4

Start of cycle

2[Pyruvate+ATP+NADH]

- Krebs Cycle- E transport chain

Aerobic condition; in mitochondriaAnaerobic

condition

Lactic Acid fermentation in muscle.Only in yeast/bacteria Anaerobic respiration or Alcohol fermentation

Pyruvate enters as AcetylcoA

Glucose

Cycle : anaerobic

GLYCOLYSIS

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GLYCOLYSIS 1st stage of glucose metabolism → glycolysis An anaerobic process, yields 2 ATP

(additional energy source) Glucose will be metabolized via gycolysis;

pyruvate as the end product The pyruvate will be converted to lactic acid

(muscles → liver) Aerobic conditions: the main purpose is to

feed pyruvate into TCA cycle for further rise of ATP

Fig. 17-1, p.464

The breakdown of glucose to pyruvate as summarized:

Glucose (six C atoms) → 2 pyruvate (three C atoms)2 ATP + 4 ADP + 2 Pi → 2 ADP + 4 ATP (phosphorylation)Glucose + 2 ADP + 2 Pi → 2 Pyruvate + 2 ATP (Net reaction)

Fig. 17-2, p.465

Louis Pasteur- French biologist- did research on

fermentation which led to important discoveries in microbiology and chemistry

HOW 6-CARBON GLUCOSE CONVERTED TO THE 3-CARBON GLYCERALDEHYDE-3-PHOSPHATE?

p.467

Step 1 Glucose is phosphorylated to give gluc-6-phosphatePreparation phase

Fig. 17-3, p.468

Table 17-1, p.469

Fig. 17-4, p.470

p.470a

Step 2 Glucose-6-phosphate isomerize to give fructose-6-phosphate

p.470b

Step 3 Fructose-6-phosphate is phosphorylated producing fructose-1,6-bisphosphate

Fig. 17-6, p.471

p.471a

Step 4 Fructose-1,6-bisphosphate split into two 3-carbon fragments

p.471b

Step 5 Dihydroxyacetone phosphate is converted to glyceraldehyde-3-phosphate

HOW IS GLYCERALDEHYDE-6-PHOSPHATE CONVERTED TO PYRUVATE

p.472

Step 6

Payoff phase

Glyceraldehyde-6-phosphate is oxidized to 1,3-bisphosphoglycerate

Fig. 17-7, p.473

p.474a

Fig. 17-8, p.475

p.476

Step 7 Production of ATP by phosphorylation of ADP

p.477a

Step 8 Phosphate group is transferred from C-3 to C-2

p.477b

Step 9 Dehydration reaction of 2-phosphoglycerate to phosphoenolpyruvate

p.478

Step 10 Phosphoenolpyruvate transfers its phosphate group to ADP → ATP and pyruvate

Fig. 17-10, p.479

Control points in glycolysis

HOW IS PYRUVATE METABOLIZED ANAEROBICALLY?

p.479

Conversion of pyruvate to lactate in muscle

Fig. 17-11b, p.481

Fig. 17-11a, p.481

Pyruvate decarboxylase

Fig. 17-12, p.482

p.482

Acetaldehyde + NADH → Ethanol + NAD+

Glucose + 2 ADP + 2 Pi + 2 H+ → 2 Ethanol + 2 ATP + 2 CO2 + 2 H2O

Carbohydrate metabolism

Chapter 3(cont.)

Gluconeogenesis

Conversion of pyruvate to glucose Biosynthesis and the degradation of many important biomolecules follow

different pathways There are three irreversible steps in glycolysis and the differences bet.

glycolysis and gluconeogenesis are found in these reactions Different pathway, reactions and enzyme

p.495

STEP 1

is the biosynthesis of new glucose from non-CHO precursors. this glucose is as a fuel source by the brain, testes,

erythrocytes and kidney medulla

comprises of 9 steps and occurs in liver and kidney the process occurs when quantity of glycogen have been

depleted - Used to maintain blood glucose levels. Designed to make sure blood glucose levels are high enough

to meet the demands of brain and muscle (cannot do gluconeogenesis).

promotes by low blood glucose level and high ATP inhibits by low ATP occurs when [glu] is low or during periods of fasting/

starvation, or intense exercise pathway is highly endergonic *endergonic is energy consuming

STEP 2

The oxalocetate formed in the mitochondria have two fates:

- continue to form PEP- turned into malate by malate dehydrogenase and leave the mitochondria, have a reaction reverse by cytosolic malate dehydrogenase

Reason?

as

Fig. 18-12, p.502

Controlling glucose metabolism• found in Cori cycle• shows the cycling of

glucose due to gycolysis in muscle and gluconeogenesis in liver

As energy store for next exercise

• This two metabolic pathways are not active simultaneously.

• when the cell needs ATP, glycolisys is more active

• When there is little need for ATP, gluconeogenesis is more active

Cori cycle requires the net hydrolysis of two ATP and two GTP.

OHATPHNADHPyruvate

PADPNADeglu i

222422

222cos

iPGDPADPNADeGlu

OHGTPATPHNADHPyruvate

6242cos

624422 2

iPGDPADPOHGTPATP

422422 2

Fig. 18-13, p.503

The Citric Acid cycle

Cycle where 30 to 32 molecules of ATP can be produced from glucose in complete aerobic oxidation

Amphibolic – play roles in both catabolism and anabolism

The other name of citric acid cycle: Krebs cycle and tricarboxylic acid cycle (TCA)

Takes place in mitochondria

Fig. 19-2, p.513

Fig. 19-3b, p.514

Steps 3,4,6 and 8 – oxidation reactions

5 enzymes make up the pyruvate dehydrogenase complex: pyruvate dehydrogenase (PDH) Dihydrolipoyl transacetylase Dihydrolipoyl dehydrogenase Pyruvate dehydrogenase kinase Pyruvate dehydrogenase phosphatase

Conversion of pyruvate to acetyl-CoA

p.518

Step 1 Formation of citrate

Table 19-1, p.518

Step 2 Isomerization

Fig. 19-6, p.519

cis-Aconitate as an intermediate in the conversion of citrate to isocitrate

Fig. 19-7, p.521

Step 3

Formation of α-ketoglutarate and CO2 – first oxidation

p.521

Step 4 Formation of succinyl-CoA and CO2 – 2nd oxidation

p.522

Step 5 Formation of succinate

p.523a

Step 6

Formation of fumarate – FAD-linked oxidation

p.524a

Step 7 Formation of L-malate

p.524b

Step 8 Regeneration of oxaloacetate – final oxidation step

Fig. 19-8, p.526

Krebs cycle produced:• 6 CO2

• 2 ATP• 6 NADH• 2 FADH2

Table 19-3, p.527

Fig. 19-10, p.530

Fig. 19-11, p.531

Fig. 19-12, p.533

Fig. 19-15, p.535

Overall production from glycolysis, oxidative decarboxylation and TCA:

Oxidative decarboxylation

Glycolysis TCA cycle

- 2 ATP 2 ATP

2 NADH 2 NADH 6 NADH , 2 FADH2

2 CO2 2 Pyruvate 4 CO2

Electron transportation system