chapter 4 carbohydrate metabolism 牛永东生物化学的学习方法...

Chapter 4 Carbohydrate Metabolism

牛永东

生物化学的学习方法特点一：与数理化不同，尚未进入定量科

学的阶段，还处在定性科学阶段。因此不可能通过公式或定理推出一个准确的结论

特点二：是没有绝对，几乎所有的结论都可以被一些例外打破（生物多样性）

一般性结论：生物化学的学习应以概念为主 --- 以记忆为主，在记忆的基础上加以理解

【目的与要求】• 掌握糖酵解 glycolysis 、有氧氧化、巴斯德效应 Pastuer

effect 、磷酸戊糖途径、磷酸戊糖途径、糖异生 gluconeogenesis 、乳酸循环 Cori cycle 等概念……

• 掌握糖酵解、有氧氧化 (TCA cycle) 、磷酸戊糖途径、糖异生、糖原合成 glycogenesis 与分解 glycogenolysis 的细胞定位、过程、关键酶、调节及意义……

• 掌握血糖的来源和去路及其调节……

outline• Introduction• Glycolysis (Anaerobic Degradation)• Aerobic oxidation of glucose• The Pentose Phosphate Pathway• Glycogen Formation and Degradati

on• Gluconeogenesis

Definition of Metabolism• Metabolism (Greek for change) : all the chemical and physical processes that

take place in the body * Synthesis (anabolism): Glucose – Glycogen FA+ Glycerol – TG Amino Acids – Protein Requires Energy

Macromolecules

Small moleculesA

NA

BO

LIS

M

CA

TA

BO

LISM

* Breakdown (catabolism): Glycogen – Glucose TG – Fatty Acids + Glycerol Protein – Amino Acids Energy is released

1. What are 1. What are Carbohydrates?Carbohydrates?

• Carbohydrates are aldehyde or ketone compounds with multiple hydroxyl groups

• Empirical formula = (CH2O)n, literally a “carbon hydrate”

• CHO make up 3% of the body’s organic matter

Functions of Functions of CHOCHO• Energy SourceEnergy Source((66.8 kJ/1g carbohydrate)

• Structural elements

• Component of nucleic acids

• Conversion to lipids and non-essential amino acids

• …….

Categories of Categories of CarbohydratesCarbohydrates

Monosaccharides (Single sugar units): the smallest carbohydrates ,serve as fuel and carbon sources

Disaccharides (formed from 2 monosaccharides joined by a glycoside linkage)

Polysaccharides: many monosaccharide units (starch, cellulose)

Monosaccharides• Glucose (CC66HH1212OO66)

-found in fruits, vegetables, honey

-“blood sugar” -used for energy• Fructose - Found in fruits, honey, corn syru

p -“fruit sugar”• Galactose - Found as part of lactose in milk

Glucose (CC6HH12OO6)

Disaccharides Sucrose = glucose + fructose (brown sugar;25% of sugar inta

ke) Lactose = glucose + galactose (milk sugar; least sweet) Maltose = glucose + glucose (honey) Natural SweetnessNatural Sweetness

MaltoseSucrose

Polysaccharides

starch CH2OH

O

O

OO

O

O

O

O

O

O

O

O

CH2OH

CH2OH CH2OH CH2CH2OH

O

-[1-6] linkage

-[1-4] linkages

Glucose 14

6O H

HH

H

OHOH

H OH

OH

CH2OH

Section 1Section 1DigestionDigestion of

carbohydrates

Starch

Glycogen Salivary

amylase

Pancreatic

amylase

maltose

sucrose

lactose

galactose

stomachBrush

Border

of the

Mucosal

Epithelium

BLOOD

glucosefructose

galactose

glucose

fructose

Monosaccharides

1

2

3

a -

amylase

Mouth

no significant

digestive

enzymes

present

Responsible for

most of

carbohydrate

digestion

limited

breakdown of

starch and

glycogen occurs

to capillaries

Na+ dependent glucose Absorption and transporter,SGLT

Lumen of intestineIntestinal Epithelial cell

Fructose; also glucose,

Glucose Galactose Na+

2K+

Na+

3Na+

ATP

ADP + Pi

= facilitated diffusion

= Na+-dependent co-transport

= Na,K-ATPase

GLUT-5

Brush border

SGLT-1

Fructose

GalactoseGlucose

GLUT-2

GalactoseGlucose

GalactoseGlucose

Na+

Na+

2K+

2K+

2K+

3Na+

3Na+

Family of glucose transporters

Name Tissue location Km CommentsGLUT1 All mammalian tissues 1mmol/L Basal glucose uptakeGLUT2 Liver and pancreatic 15~20mmol/L In the pancreas,plays a role cells in regulation of insulin In the liver, removes excess glucose from the bloodGLUT3 All mammalian tissues 1mmol/L Basal glucose uptakeGLUT4 Muscle and fat cells 5mmol/L Amount in muscle plasma membrane increases with endurance training GLUT5 Small intestine － Primarily a fructose transporter

Not digested: : Dietary Dietary FiberFiber

Water insoluble fibers- Cellulose, hemicellulose , pectins （果胶）……

Water soluble fiber- beans, rice, carrots, fruits……- Obesity, diabetes, cancer……

Recommended intake of fiber20-35 g/day; insoluble:soluble = 3:1

Anaerobic degradation (glycolysis)

glucose

Aerobic oxidation

Glycogen

UDPG

G-1-P

G-6-P 6-phosphogluconate

Fructose 6-p

trioses phosphate non-carbohydrates

pyruvate

acetyl CoAlactate

Tricarboxylic acid cycle

2 2CO+ HO + energy

Gluconeogenesis

Glycogenesis Glycogenolysis

Pentose phosphate pathwayPi

Pi

3. Overview of carbohydrate metabolism

Section 2 Glycolysis (Anaerobic Degradation)

• “Glycolysis” is derived from Greek words glycos (sugar, sweet) and lysis (dissolution)

• The glycolytic pathway (Glucose to pyruvate) was elucidated by 1940, largely through the pioneering contributions of Gustav Embden.so glycolysis is also known as the Embden-Meyerhof pathway

Glycolysis

• Where in cell ?• What are the inputs ?• What are the outcomes ?• Oxygen required ?

Glycolysis （糖酵解）• For glycolysis, the overall goal is to break th

e glucose molecule into smaller, more oxidized pieces

• 11 steps metabolic pathway to convert 6 carbon glucose into 2 molecules of 3 carbon lactate 乳酸and two molecules each of and ATP

• Occurs in cytoplasm• glycolysis has has two two stages: glycolytic pathwa

y (Glucose to pyruvate); Fermentation （发酵） phase (pyruvate to lactate)

• Anaerobic– Does not REQUIRE oxygen– Occurs whether oxygen is available or not

O

CH2OHC1

C2 H

C

C OH

C HHO

C OHH

C OHH

CH2OH

C3

C4

C5

C6

D-glucose

or

OH

D-glucose

breakC3-C4 bond

breakage of C3-C4

bond

glycolytic pathway = breakdown of glucose to yield energy and pyruvate

glycolytic pathway has has two two phases

A. Energy investment phase (Reactions, 1-5) Glucose(6C) is first phophorylated (thus acti

vated) and then cleaved to produce two glyceraldehyde-3-phosphate(3C) intermediates. 2 ATPs are invested. (the preparatory phase)

B. Energy payoff phase (Reactions 6-10) two glyceraldehyde 3-phosphate intermediates

are oxidized, generating to two pyruvate plus four ATP molecules

CH2 O H

HO-C-H

H-C-OH

CHO

H-C-OHH-C-OH

-P-O

O

OH

C=OCH2 OH-P-O

O

OH

glucoseglucose

-P-O

O

OH isomerase

Energy investment phase

Step 3. Phosphofructokinase -1 (PFK-1) (2 ATP utilizati

on)

Step 1. Hexokinase (1 ATP utilization)

Step 2. Phosphoglucose Isomerase (PGI)

H

C H2 OH

HO-C-H

H-C-OH

CHO

H-C-OHH-C-OH

C=OCH2 OH-P-O

O

OH

-P-O

O

OH

HO-C-HC=OCH2O-P-O

O

OH

-P-O

O

OHCH2O

H-C-OHH-C-OH

+H-C-OH

CH2OPO3=

HC=O

Glyceraldehyde 3-PO4

DHAP

CH2OPO3=

C=OCH2OH

dihydroxyaceto

ne phosphate

energy investment phase

4. Aldolase

isomer

ase

dihydroxyacetone phospha

teTPITPI

The isomerization of an aldose to a ketose

Glyceraldehyde 3-P

O4

energy investment phase

5. Triose Phosphate Isomerase (TIM or

TPI )

Glucose

Glucose 6-phosphate

Fructose 6-phosphate

Fructose 1.6-bisphosphate

Dihydroxyacetone phosphate Triose phosphate isome

rase

Aldolase

Phosphofructokinase

Phosphogluco- isomerase

HexokinaseATP

ADP

ATPADP

Glyceraldehyde 3-phosphate

energy investment

phase

Uses 2 ATP

CHO

H-C-OH

CH2OPO3

COO

H-C-OH

CH2OPO3

C

H-C-OH

CH2OPO3

~OPO3

OPO4

NAD+

NADH+ H+

ADP ATP

Glyceraldehyde-3-PO4 dehydrogenase

Phosphoglycerate kinase

(Reactions, 6-10)Energy payoff phase

COO

H-C-OH

CH2OPO3

3-PGA

COO

C=O

CH3

Pyruvate

COO

C~

CH2

OPO3

PEP

COO

H-C-OPO3

CH2OH

2-PGA

Low energyHigh energy

-H2O

ADP ATP

Energy payoff phase

Glyceraldehyde 3-phosphate

NAD+ + Pi

NADH + H+

1,3-Bisphosphoglycerate

Glyceraldehyde 3-phosphate dehydrogenase

Phosphoglycerate kinase ADP

ATP 3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate

Pyruvate kinase ADPATP

H2OEnolase

Phosphoglyceromutase

energy payoff phase

ATP

generation

Oxidation

ATP generation

Overview of glycolytic pathway

Summary of Energy Relationships for glycolytic

pathway * Input = 2 ATP 1. glucose + ATP glucose-6-P 2. fructose-6-P + ATP fructose 1,6 bisphosphate* Output = 4 ATP + 2 NADH1. 2 glyceraldehyde-3-P + 2 Pi + 2 NAD+ 2 (1,3 bisphosphoglycerate) + 2 NADH2. 2 (1,3 bisphosphoglycerate) + 2 ADP 2 (3-P-glycerate) + 2 ATP3. 2 PEP + 2 ADP 2 pyruvate + 2 ATP

*Net = 2 ATP and 2 NADH

Fate of Pyruvate• Two anerobic pathways: (Low O2 ) - to lactate via lactate dehydrogenase in muscle - to ethanol (fermentation) via ethanol dehydro

genase• Aerobic pathway – through citric acid cycle and

respiration; Enough O2,this pathway yields far more energy

NADH + O2 NAD+ + energy Pyruvate + O2 3CO2 + energy

AnaerobicAnaerobicGlycolysisGlycolysis

PyruvatPyruvateeAlcoholAlcohol

FermentatioFermentationn

Aerobic GlycolysisAerobic Glycolysis

Oxygen availability determines fate of PyrPyr

uvateuvate

The anaerobic fate of Pyruvate ( Reaction 11 of glycolysis )

• Hydrogen at C4 of NADH is transferred to the pyruvate

• uses up all the NADH (reducing equivalents) produced in glycolysis

Energy Yield From GlycolysisEnergy Yield From Glycolysis

• glucose 6 CO2 = -2840 kJ/mol• 2 ATPs produced = 61 kJ/mol glucose• Energy yield = 61/2840 = 2% in all: high investment, low output

Overall process of anaerobic glycolysis in muscle can be represented:

The lactate, the end product, is exported from the muscle cell and carried by the blood to the liver, where it is reconverted to glucose

a. 11 steps ; Location: cytosolb. Original material: glucose (C6H12O6) c. End product: lactate - Twice substrate level phosphorylations - Net of 2 ATP d. Key enzymes: Hexokinase (HK) …….energy investment

phase

Phosphofructokinase 1 (PFK-1) …….energy investment phase

Pyruvate kinase (PK) …….energy payoff phasee. Once dehydrogenation: oxidation Once hydrogenation: reductionf. No oxygen is required

Summary of Glycolysis

The regulation of glycolysisHormone regulationCovalent regulationAllosteric regulation

GlucagonATP

cAMP

ATP ADP

F-6-P F-2,6-BP

PFK-2active

FBP-2inactive

P P

FBP-2 active inactive

PFK-2

Pi

PKA

ATP

ADP Pi

Phosphoprotein Phosphatase

F-1,6-BP

Glucose

ATP

ADP

PFK-1

AMP Citrate

AMP Citrate

－

－

－

Lactate

Adenylate cyclase

Glucose 6-phosphate

glycolysis

3. The significance of glycolysis

• Glycolysis is the emergency energy-yielding pathway, such as play ball, climb mountain.…..• Glycolysis is the major way to produce ATP in some tissues, even though the oxygen supply is sufficient, such as RBC, retina, testis, skin……

Section 3 Aerobic oxidation of glucose

• The process of oxidation completely

from glucose to CO2 and H2O is named

aerobic oxidation

• This process is the major process to provide energy for most tissues

3 phases of Glucose Aerobic oxidation

2. Oxidation from pyruvate to acetyl CoA in mitochondria (3C to 2C) 3. Tricarboxylic acid cycle and oxidative phosphorylation (2C to 1C)

O2O2 O2

Glucose G-6-P

PyruvatePyruvate

Acetyl CoA

TCA cycle

H+ +e

CO2

H2O

cytosol mitochondria

1. Oxidation from glucose to pyruvate in cytosol (6C to 3C)

CoASHCoASH

CH3

CO C

O-

O

CO2CH3

CO S

CoA

NAD+NAD+

Pyruvate DH comPyruvate DH complexplex

2. Oxidation from pyruvate to acetyl CoA (3C to 2C)

Pyruvate+NAD++HSCoA

Acetyl CoA+NADH+H++CO2

NADH

Acetyl-CoA: a common two-carbon unit

E1. pyruvate dehydrogenase （丙酮酸脱氢酶）E2. dihydrolipoyl transacetylase（二氢硫辛酰胺转乙酰酶）

E3. dihydrolipoyl dehydrog

enase

（二氢硫辛酰胺脱氢酶）

Pyruvate dehydrogenase complex


TPPE1

FADE3

E2

3. Two stages of the 3rd phase of Glucose

Aerobic oxidation• Stage I The acetyl-CoA is completely oxidize

d into CO2, with electrons collected by NAD and FAD via a cyclic pathway (tricarboxylic acid cycle)

• Stage II Electrons of NADH and FADH2 are transferred to O2 via a series carriers, producing H2O and a H+ gradient, which will promote ATP formation （ oxidative phosphorylation ）

(NEXT CHAPTER)

Tricarboxylic acid cycle (2C to 1C)• Citric Acid Cycle or Krebs cycle• Occurs in mitochondrial matrix• Is the biochemical hub of the cell, oxidizing car

bon fuels, usually in the form of acetyl CoA, interconversion of carbohydrates, lipids, and some amino acids, as well as serving as a source of precursors for biosynthesis

• For the citric acid cycle, the goal is to use the oxidative power of O2 to derive as much energy as possible from the products of glycolysis

NADH+H+

CO2

C2

NADH+H+

CO2GTP

FADH2

NADH+H+

C6

C4

C4

C5


Substrates required: Oxaloacetic Acid GDP 3NAD+ FAD two-carbon units (Acetyl-CoA) Intermediate Reactants: Citric Acid

Output: Oxaloacetic Acid GTP 3 NADH FADH2 2CO2

(4 high-energy electrons)

Each Acetyl-CoA yields 2 CO2, 3 NADH + H+, 1 FADH2, 1 GTP

CO C

CH2

C

O

O-

-O O

CH3

CO S

CoA

Citrate synthaseCitrate synthase

CoASH

Stage I Tricarboxylic acid cycle

Oxaloacetic Acid

＋＋C

CH2

C3

HO C2

CH2

C1

O O-

O

O-

-O O

Citrate4C

2C

6 C

AconitaseAconitase

ciscis-aconitate inter-aconitate intermediatemediate

C

H2C

C

C

CC

O

O-

O

O-

-O

OH

C

CH2

C3

HO C2

CH2

C1

O O-

O

O-

-O O6 C

6 C

Isocitrate DH

O-

NADNADH

a-ketoglutaratea-ketoglutarateIsocitrate

CH2

CH2

C

CC

O O-

-O

OO

CO2

C

CH2

C

H C

CC

O

O

O-

-O

OHO

6 C

5 C

a-ketoglutarate DHa-ketoglutarate DH

NADNAD++, , CoASHCoASH NADNADHHNADNADHH

Succinyl CoASuccinyl CoAa-ketoglutaratea-ketoglutarate

CH2

CH2

C

CC

O O-

-O

OO

CH2

CH2

C

CS

O O-

CoAO

CO2

5 C

4 C

GDP, PGDP, PiiGTPGTP

Succinyl CoASuccinyl CoA

SuccinylCoA synthetaseSuccinylCoA synthetase

CH2

CH2

C

CS

O O-

CoAO

CH2

CH2

C

CO-

O O-

O

CoASH

4C

4C

(FAD)(FAD)

fumaratefumarate

(FADH(FADH22))(FADH(FADH22))CH2

CH2

C

CO-

O O-

O

C

C

C

C

O-

O

O-

O H

H

4C

4C

HH22OO

fumaratefumarate malate

C

C

C

C

O-

O

O-

O H

H

CHO

C

CH2

C

O O-

-O O

H

4C

4C

malate DH

NADNAD++ NADHNADHNADHNADH

malate Oxaloacetic Acid

CHO

C

CH2

C

O O-

-O O

H CO C

CH2

C

O

O-

-O O

4C4

C


CH COOH

CH2 COOH

HO

CH2 CO O H

HC C O O H

C H C O O HHO

NAD+

NADH+H+

C O2

CH2 CO O H

C H2

C C O O HO

CH3 C

O

~SCoAO C C O O H

C H2

C O O H

CH2 COOH

C COOH

CH2

HO

COOH

CH2 CO O H

C C O O H

C H CO O H

CoASH

GTPCoASH

GDP+Pi

CH2 COOHCH2 COOH

FAD H2

FAD

HC COO H

CH

HOOC

CH2 CO O H

C H2

CO S C o A~CoASH C O2

NAD+NADH+H+

NAD+NADH+H+ citrate synthasecitrate synthase

isocitrate dehydrogenase


- ketoglutarate dehydrogenase- ketoglutarate dehydrogenase

Aerobic oxidation of glucose

C6H12O6 + 6O2 + 38 ADP +38 P 6CO2 + 6H2O + 38 ATP

Generation of ATP in aerobic oxidation of glucose

Total per mole of glucose under aerobic conditions: 38 or 36 (32 or 30) ATPs

Reactions Catalyzed by

Methods of ATP production formed moles of ATP

Glyceraldehyde 3-phosphate dehydrogenaseGlycolytic

pathway

Respiratory chain Oxidation of 2 NADH 6 or 4 /5 or 3

Phosphoglycerate kinase Phosphorylation at substrate level 2Pyruvate kinase Phosphorylation at substrate level 2

consumption of ATP by reactions catalyzed by hexokinase and phosphofructokinase- 2

Production ofacetyl CoA


Respiratory chain Oxidation of 2 NADH

TCA cycle

Isocitrate dehydrogenaseAlpha-ketoglutarateDehydrogenase complex

Succinyl CoA synthetaseSuccinate dehydrogenaseMalate dehydrogenase



Phosphorylation at substrate level

Respiratory chain Oxidation of 2 FADH2


6 or 5

6 or 5

4 or 3

6 or 5

2

6 or 5

3 control points of citric acid cycle :

citrate synthase


-ketoglutarate dehydrogenase

C2

C 6

C4

C4

C5

Krebs Cycle

pyruvate

Iso C6

Regulation of pyruvate dehydrogenase

Regulation of aerobic oxidation

Regulation of pyruvate dehydrogenase

Inhibited by products,NADH & Acetyl CoA

Also regulated by covalent modification,the kinase & phosphatase also regulated

GTP

Acetyl CoA

Oxaloacetate

CoA

Citrate [1]

cis-Aconitate

Isocitrate

-Ketoglutarate

NAD+

NADH, CO2

[2]

Succinate GDP

[4] Fumarate

FAD

FADH2

[5]

Malate

NAD+

NADH

[6]

CoA, NAD+

[3]

Succinyl CoA NADH, CO2

Allosteric inhibitor

ADPallosteric acti

vator

NADH:allosteric inhibitor

GTPallosteric inhibitor

The Alosteric regulation of citric acid cycle citrate synthase


- ketoglutarate dehydrogenase

* Pastuer effect• The total amount of glucose consumed by yeast are

about 7 times greater under anaerobic conditions than under aerobic conditions

• This effect is also seen in muscle under anaerobic conditions

• The yield of ATP under anaerobic conditions is 2 per molecule; but under aerobic conditions the yield is 38 ATP per glucose

• Therefore, glucose flux through the pathway is regulated to achieve constant ATP levels or decided by the fate of NADH+H+

Section 4

The Pentose Phosphate

Pathway (PPP)

Pentose phosphate pathway

• The PENTOSE PHOSPHATE pathway ,by carring out oxidation and decarboxylation of the 6-C sugar glucose-6-P, is basically used for the synthesis of NADPH and 5-C sugar ribulose-5-P

• It plays only a minor role (compared to GLYCOLYSIS) in degradation for ATP energy

• Other names: Pentose phosphate Shunt Hexose Monophosphate Shunt

• Two stages:– Oxidative portion (NADPH producing)– Non-oxidative (carbon recycling/unit transfe

rring)• Location: cytosol• Original material: glucose 6-phosphate• End product: NADPH , pentose phosphate• Important in adipose tissue, adrenal cortex, live

r (biosynthesis) 、 Important in red blood cells (antioxidant reasons)


G-6-P dehydrogenase

CHO

C-OH

HO-C

C-OHC-OH

CH2OP

COO-

C-OH

HO-C

C-OHC-OH

CH2OP

C

C-OH

HO-C

CC-OH

CH2OP

O

O

NADP+

NADPH + H+

+ H2O

STAGE I(Oxidation=NADPH producing and formation of pentose phosphate)

Glucose-6-PO46-Phosphoglucono-- lactone

6-Phospho- gluconate

Lactonase

G-6-P dehydrogenase: Rate limiting step, controlled by NADP+ levelsGlucose-6-phosphate Dehydrogenase catalyzes oxidation of the aldehyde (hemiacetal), at C1 of glucose-6-phosphate, to a carboxylic acid, in ester linkage (lactone). NADP+ serves as electron acceptor

COO-

C-OH

HO-C-H

C-OHC-OH

CH2OP

C=OCH2OH

C-OHC-OH

CH2OP

CHO

C-OHC-OH

CH2OP

C-OHCO2

NADP+

NADPH + H+

Ru5P

isomeraseRu5P

epimerase

Ribose-5-PO4

5-p- 木酮糖Xylulose-5-PO4

Ribulose-5-PO4 (Ru5P)

6-Phosphogluconate

6-phosphogluconateDehydrogenase

CH2OH

C-OHC

CH2OP

C=O

HO-

STAGE I

6C

5C

5C

5C

CH2OH

C-OHC

CH2OP

C=O

HO-

CHO

C-OHC-OH

CH2OP

C-OH

C-OHC-OH

CH2OP

C-OHHO-C

C=O

CH2OH

+

C-OH

CH2OP

CHO

酮醇转移酶

Transketolase

7-p- 景天糖Sedoheptulose-7-PO4

Ribose-5-PO4Xyulose-5-PO4

Glyceraldehyde-3-PO4

STAGE II ( Non-oxidative=carbon recycling)

5C

5C

7C

3C

C-OH

CH2OP

CHO

C-OHC-OH

CH2OP

C-OHHO-C

C=O

CH2OH

C-OHC-OH

CH2OP

CHO

C-OHC-OH

CH2OP

HO-CC=O

CH2OH

+


Fructose-6-PO4

4-p- 赤藓糖Erythrose-4-PO4

Transaldolase醛糖移转酶

Sedoheptulose-7-PO4


3C

7C

4C

6C

C-OHC-OH

CH2OP

CHOCH2OH

C-OHC

CH2OP

C=O

HO-+ C-OH

CH2OP

CHO

C-OHC-OH

CH2OP

HO-CC=O

CH2OH

+

Transketolase

Erythrose-4-PO4Xylulose-5-PO4


Fructose-6-PO4


glycolysis

4C

5C

3C

6C

Sedoheptulose 7-P

(7 C)Erythrose 4-P (4C)

Ribose 5-P (5 C)Glyceraldehyde 3-P

(3C)

Fructose 6-P

(6C)

Fructose 6-P

Glyceraldehyde 3-P

GlucoseATP ADP

Glucose 6-P 6-Phosphogluconate

Oxidative stage

Xylulose 5-P ( 5C)

Ribulose 5-P (5C)

CO2NADPH

NADP

NADP NADPH

Non

-oxid

ativ

e

stag

e


C5 + C5 C3 + C7 (Transketola

se)

C3 + C7 C6 + C4 (Transaldola

se)

C5 + C4 C6 + C3 (Transketola

se)

3 C5 2 C6 + C3 (Overall)

+

+ +

+

+

SUMMARY

3CO2

3 G 6-P C5 + C5 + C5 C3 + C6+ C6

3 Glucose-6-PO4 + 6 NADP+ + 3H2O6 NADPH + 6H+ + 3CO2 + 2 Fructose-6-PO4

+ Glyceraldehyde-3-PO4•

Per glucose oxidized, 2 NADPHs are formed

•C7 、 C4 are strictly intermediates

•Glyceraldehyde-3-PO4 is both an intermediate and finalproduct

•Fructose-6-PO4 is never used as an intermediate, return to the glycolytic p

athway

*** The significance of PPP1. Produce ribose 5-phosphate needed for DNA and RNA synthesis2. Generate reducing equivalents NADPH 1) Reducing power for biosynthesis of fatty

acids, cholesterol, folate, and so on 2) Coenzyme of glutathione reductase to k

eep the normal level of reduced glutathione

3) NADPH serves as the coenzyme of mixed function oxidases (mono-oxygenases)

Section 5

Glycogen Biosynthesis and Degradation

Introduction

A constant source of blood glucose is an absolute requirement for life - glucose is the preferred energy source for the brain and for cells with few or no mitochondria, such as mature erythrocytes - glucose is an essential energy source in exercising muscle

1. Glycogen is a highly branched homopolymer of a-glucose (polysaccharide)2. Approx. every 10 residues there is a branch, linked by an a-1,6-glycosidic linkage

What is Glycogen?What is Glycogen?

Reducing

end

CH2OH

O

CH2O

O

CH2OH

O

CH2OH

O

CH2OH

O

CH2OH

O

a-1,4-glycosidic linkage

a-1,6-glycosidic linkage ））

Glycogen Biosynthesis(Glycogenesis)

• Glycogenesis: the process of storing excess glucose as glycogen (In times of plenty the body needs to store fuel)

• occurs in the cell cytoplasm of liver, muscle & kidney, when blood glucose levels are high

• Excess glucose is stored (limited capacity)– liver and muscle are major glycogen storage sites

• liver glycogen used to regulate blood glucose levels– brain cells cannot live for > 5 minutes without gl

ucose• muscle glycogen used to fuel an active muscle

• Glycogenesis involves addition of a-D-glucose residues to the C4 (non-reducing end) of an pre-existing chain

Requirement of formation glycogen

• Glycogenin

• glycogen synthase

• glycogen-branching enzyme

• UDP-glucose pyrophosphorylase

Requirement of Formation glycogen

• Primer– glycogenin acts as the primer to whi

ch the first glucose residue is attached

– glycogenin also catalyzes attachment of additional glucose units to form chains of up to eight units

• Glycogen-branching enzyme

– takes over at this point – chain cannot extend indefinitely

Requirement of Formation glycogen

• Glycogen Synthase– exists in an active (dephosphorylated) and inactive (p

hosphorylated) form– relative amount of each form is regulated by cellular l

evel of cAMP– cAMP is regulated by insulin:glucagon ratio

• High insulin keeps GS in dephosphorylated, active form• High insulin can also stimulate dephosphorylation of GS• High glucagon activates cAMP which activates PK which pho

sphorylates and inactivates GS– Glycogen Synthase reaction is primary target of insuli

n’s stimulatory effect on glycogenesis

glucose G-6-PGK(liver),HK

ATP ADPMg2+

phosphoglucomutase

G-1-P

UTP (uridinetriphosphate) PPi

UDPG (uridinediphosphate glucose)

H2O

2Pi

OCH2OH

PO

O-

O

O-

G-1-P

+ OP-O

O-

O

OP

O-

O

NH

O

ON

O

OHOH

HH

CH2

H

OP

O-

O

UDPG pyro-phosphorylase

OCH2OH

PO

O-

O

NH

O

ON

O

OHOH

HH

CH2

H

OP

O-

O

O

UDPG

GLYCOGEN SYNTHASE

UDP

glycogen¦Á-1,4-glycosidic bond

O

O-UDP

CH2OH

UDPG

+O

CH2OH

O

O O

CH2OH

glycogen primer (n)

UDP

O

O

O

O

O

O

CH2OH CH2OH CH2OH

glycogen (n+1)

GLYCOGEN SYNTHASE

branching enzyme

glycogen¦Á-1,4,¦Á-1,6-glycosidic bond

glycogen primer

UTP

PPi

Glycogenesis

glycogen synthase

oligo 1,6-glucantransferase Debranching

glycogen

synthase

oligo 1,6-glucantransferase Debranching

Glycogenesis

GlycogenolysisGlycogenolysis : glycogen glucose

• In times of need the body needs to mobilize its’ fuel stores

• Hepatic glycogen not sufficient during 12 hr fast• Glycogen degradation• Occurs in cytosol• Signal that glucose is needed is given by hormones

– epinephrine stimulates glycogen breakdown in muscle

– glucagon which stimulates glycogen breakdown in liver in response to low BG

– used to sustain blood glucose level between meals and to provide energy during an emergency/exercise

Glycogenolysis

1,4 glucose 1-phosphate

12 glucose 1-phosphate

1 glucose

phosphorylase a

phosphorylase aglucan transferase

glucosidase

phosphorylase a

Debranching has two enzyme activities in one peptide: oligo -1,4 1,4-glucantransferase and 1,6-glucosidase

Glycogen Phosphorylase Regulation

• Glycogen phosphorylase– exists in a “b” inactive form (dephosphorylat

ed) and an “a” active form (phosphorylated)– phosphorylase kinase converts glycogen phos

phorylase to active form “a” via addition of inorganic phosphate • phosphorylase kinase also exists in an active “a”

and an inactive “b” form– activated by cAMP-dependent protein kinase; it is also acti

vated by calcium ions– PK is activated by glucagon and epinephrine

» via 2nd messenger cAMP

Glycogen Phosphorylase Regulation

• Glycogen phosphorylase “a” (active) is converted to “b” form by phosphoprotein phosphatase– Stimulated by insulin

• Glycogen phosphorylase can also be regulated by allosterically– GP “b” inactive form can be converted to GP

“b” active form by high AMP – GP “b” active form can be converted back to GP

“b” inactive form by high ATP

hormons:glucagon, epinephrine

inactiveadenylate cyclase

ATP

ATP

ATP

ATP

cAMP

inactiveproteinkinase A

active protein kinase A

ADP

ADPADP

phosphorylase b kinase

phosphorylase b kinase

P

Pi

Pi

Pi

H2O

H2O

H2O

phosphorylase b phosphorylase a

P

glycogen synthase(active)

glycogen synthase

P

(inactive)

protein phosphatase-1

inhibitor-1 (inactive)

Pinhibitor-1 (active)ATP

glycogenolysis

glycogenesis

active adenylate cyclase

Regulation of Glycogenesis and Glycogenolysis

The significance of glycogenesis and glycogenolysis- Liver glycogen (as much as 10% of liver wet weight) functions as a glucose reserve for maintaining blood glucose concentration- Muscle glycogen (total 400 gram) serves as a fuel reserve for synthesis of ATP within that tissue

Section 6

Gluconeogenesis

*** Gluconeogenesis• Synthesis of glucose from non-CHO precursors

– Lactate, most amino acids and glycerol ***– Lactate and amino acids (except leucine and

lysine) are converted to either pyruvate or OAA (oxalloacetate)

– Glycerol is converted (phosphorylated) to G3P and then to dihydroxyacetone phosphate

• Occurs primarily in liver, sometimes kidney

Blood Glucose

Ribose 5-PO4Ribose 5-PO4

GlycogenGlycogen

L-lactateL-lactate Pyruvate

PEP

2PGA

3PGA

1,3bisPGA

Gly-3-P

F1,6bisP

OAA

F6P

G6P

DHAP

Glucose

H2O

PO4

Phosphatase

H2OPO4Phosphatase

Kinase

Kinase

Kinase

KinaseGluconeogenesis

Gluconeogenesis• Reversal of glycolysis except at 3 steps

– HK(GK), PFK and PK• 3 Steps need to be bypassed

Hexokinase and Phosphofructokinase are bypassed by glucose 6 phosphatase and fructose 1,6-bisphosphatase

Pyruvate Kinase bypass involves formation of OAA as an intermediate

– OAA in mitochondrial matrix cannot directly cross membrane so is converted to malate

Gluconeogenesis• Bypass of PK reaction continued

– Malate and aspartate can transverse mitochondrial matrix• converted back to OAA in cytoplasm

– OAA is decarboxylated and phosphorylated to PEP by PEP carboxykinase

• Carbon skeletons of many amino acids that enter TCA cycle can thus be used for glucose synthesis (glucogenic amino acids)

Bypasses in Gluconeogenesis-1 (2 reactions)Pyruvate Carboxylase (Gluconeogenesis) catalyzes:

pyruvate + HCO3- + ATP oxaloacetate + ADP + Pi

PEP Carboxykinase (Gluconeogenesis) catalyzes:oxaloacetate + GTP PEP + GDP + CO2

C

C

CH2

O O-

OPO32-

C

C

CH3

O O-

O

ATP ADP + P i C

CH2

C

C

O

O O-

O-

O

HCO3-

GTP GDP

CO2

pyruvate oxaloacetate PEP

Pyruvate Carboxylase PEP Carboxykinase

Fructose 1,6 bi-sphosphatase

glycolysis

ATPADP

Fructose 1,6-2 PO4 Fructose-6-PO4

H2O PO4

Gluconeogenesis

Mg2+

Bypasses in Gluconeogenesis-2

Glucose-6-Phosphatase (Gluconeogenesis) catalyzes: glucose-6-phosphate + H2O glucose + Pi

H O

OH

H

OHH

OH

CH2OH

H

OH

HH O

OH

H

OHH

OH

CH2OPO32-

H

OH

HH2O

1

6

5

4

3 2

+ P i

glucose-6phosphate glucose

Glucose 6 phosphatase

Bypasses in Gluconeogenesis-3

Substrate cycle or futile cycle: nothing is accomplished but the waste of ATP. In substrate cycle, ATP is formed in one direction and then is hydrolyzed in the opposite direction. Substrate cycle produces net hydrolysis of ATP We must remember that the direction of the substrate cycle is strictly controlled by allosteric effectors to meet the needs of the body for energy

Substrate cycle is a pair of opposed irreversible reactions

Glucose Paradox

• Evidence that glucose ingested during a meal is not used to form glycogen directly

• Glucose is first taken up by RBCs in bloodstream and converted to lactate by glycolysis

• Lactate is taken up by liver and converted to G6P by gluconeogenesis

• G6P converted to glycogen

1. To keep blood sugar level stable 2. To replenish liver glycogen3. To clear the products of other tissues’ metab

olites from the blood4. To convert glucogenic amino acids to glucose5. To regulate acid-base balance

H+

phosphoenolpyruvate carboxykinase

induces biosynthesisgluconeogenesis

alpha-ketoglutarate glucose

glutamic acid

glutamine

NH3

NH3

NH4+

H+

NH4+excreted in

urine and pH

raised in blood

Na+ absorbed

urine

The significance of gluconeogenesis

Cori Cycle （ Muscles lack G6-phosphatase ） - used to prevent high blood lactate levels and to fuel muscle activity - l-actate leaves muscle cells - transported via blood to liver - liver converts to glucose - glucose released back into circulation - returned to muscles

F-6-P

F-1,6-BP

phosphofructokinase-1F-1,6-biphosphatase

ATPcitrateADPAMP F-2,6-BPF-1,6-BP

insulin

glucokinasephosphofructokinase-1

pyruvate kinase

glycolysis gluconeogenesis

pyruvate carboxylasephosphoenolpyruvate carboxykinasefructose 1,6-biphosphatase

glucose 6-phosphatase

glucagon Glucocorticoidsepinephrine

Regulation of gluconeogenesis and glycolysis

***** Section VII Blood Sugar and Its Regulation

Blood sugar3.89~6.11m

mol/L

Origin (income)

Fate (outcome)

Dietary

supply Liver

glycogenGluconeoesis

(non-carbohydrate)Other sacchari

des8.89-- 10.00mmol/L

(threshold of kidney)

glycogen

glycogenesis

urine glucose

aerobic oxidationCO2 + H2O + energy

PPPother saccharides

non-carbohydrates(lipids and some amino acids)

glycogenolysis

1

2

2

3

4

55

6

Regulation of high

Blood Sugarinsulin

High blood sugar level (hyperglycemia)

insulin receptor

active transportin muscle and adipose tissuecells (not in liverand brain)

glycolysisand aerobicoxidation

glycogenesisproteinsynthesis

lipogenesislipolysis

gluconeogenesismodulating systemcAMP

Low blood sugar level (hypoglycemia)

glucagon

cAMP

Modulating system

hepatic glycogenolysis

gluconeogenesis lipolysis transport of glucogenic amino acids

glycolysis

hepatic glycogenesis

1 1

3 342

Regulation of Low Blood

Sugar

选择题练习糖代谢

1. 糖类最主要的生理功能是 ( )

A 提供能量

B 细胞膜组分

C 软骨的基质

D 信息传递

E 免疫作用

2. 关于糖类消化吸收的叙述 , 错误的是 ( )

A 食物中的糖主要是淀粉

B 消化的部位主要是小肠

C 部分消化的部位可在口腔

D 胰淀粉酶将淀粉全部水解成葡萄糖

E 异麦芽糖酶可水解 -1,6- 糖苷键

3. 关于糖酵解途径中的关键酶正确的是 ( )

A 磷酸果糖激酶 -1

B 果糖双磷酸酶 -1

C 磷酸甘油酸激酶

D 丙酮酸羧化酶

E 果糖双磷酸酶 -2

4. 1 分子葡萄糖在有氧或无氧条件下经酵解途径氧化产生 ATP 分子数之比为 ( )

A 2

B 4

C 6

D 19

E 36

5. 1 分子乙酰 CoA 经三羧酸循环氧化后的产物是 ( )

A 柠檬酸

B 草酰乙酸

C 2CO2+ 4 分子还原当量

D CO2+H2O

E 草酰乙酸 +CO2

6. 三羧酸循环主要在细胞的哪个部位进行 ?

A 胞液

B 细胞核

C 微粒体

D 线粒体

E 高尔基体

7. 磷酸戊糖途径是在哪个亚细胞部位进行的 ?

A 胞液中

B 线粒体

C 微粒体

D 高尔基体

E 溶酶体

8. 磷酸戊糖途径主要的生理功用 ( )

A 为核酸的生物合成提供核糖

B 为机体提供大量 NADPH+H+

C 生成 6- 磷酸葡萄糖

D 生成 3- 磷酸甘油醛

E 生成 6- 磷酸葡萄糖酸

9. 关于糖原合成的叙述错误的是 ( )

A 葡萄糖的直接供体是 UDPG

B 从 1- 磷酸葡萄糖合成糖原不消耗高能磷酸键

C 新加上的葡萄糖基连于糖原引物非还原端

D 新加上的葡萄糖基以 -1,4 糖苷键连于糖原引物上

E 新加上的葡萄糖基连于糖原引物 C4 上

10. 下例哪种酶不是糖异生的关键酶 ?

A 丙酮酸羧化酶

B 磷酸烯醇式丙酮酸羧基酶

C 磷酸甘油酸激酶

D 果糖双磷酸酶

E 葡萄糖 6- 磷酸酶

11. Which one is the main organ that regulate blood sugar metabolism?

A brain

B kidney

C liver

D pancreas

E adrenal gland

12. The end product of glycolytic pathway in human body is ( )

A CO2 and H2O

B pyruvic acid

C acetone

D lactic acid

E oxalacetic acid

13. Which one can promote synthesis of glucogen, fat and protein simultaneously?

A glycagon

B insulin

C adrenaline

D adrenal cortex hormone

E glucocorticoid

14. Which one is the allosteric inhibitor of 6-phosphofructokinase-1?

A 1,6-diphosphofructose

B 2,6 -diphosphofructose

C AMP

D ADP

E citric acid

15. 关于糖酵解的叙述 , 下列那些是正确的 ?

A 整个过程在胞液中进行

B 糖原的 1 个葡萄糖单位经酵解净生成 2 分子 ATP

C 己糖激酶是关键酶之一

D 是一个可逆过程

E 使 1 分子葡萄糖生成 2 分子乳酸

16. 三羧酸循环中 , 不可逆的反应有 ( )

A 柠檬酸 → 异柠檬酸

B 异柠檬酸 → -酮戊二酸

C -酮戊二酸 → 琥珀酰 CoA

D → 琥珀酸延胡索酸

E → 苹果酸草酰乙酸

17. 如果摄入葡萄糖过多 , 在体内的去向是 ( )

A 补充血糖

B 合成糖原储存

C 转变为脂肪

D 转变为唾液酸

E 转变为非必需脂肪酸

18. 胰岛素降血糖的作用是 ( )

A 促进肌肉脂肪等组织摄取葡萄糖

B 激活糖原合成酶促糖原的合成

C 加速糖的氧化分解

D 促进脂肪动员

E 抑制丙酮酸脱氢酶活性

19. The cofactors of pyruvic dehydrogenase complex is ( )

A thioctic acid

B TPP

C CoA

D FAD

E NAD+

20. The high-energy compounds produced by substrate level phosphorylation in glyco-aerobic oxidation are ( )

A ATP

B GTP

C UTP

D CTP

E TTP

chapter 4 carbohydrate metabolism 牛永东 生物化学的学习方法...

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chapter 4 carbohydrate metabolism 牛永东生物化学的学习方法...