Storage

Pentose phosphate pathway (oxidation)

Glycolysis (oxidation)

Glycogen, Starch, Sucrose

PyruvateRibose 5-phosphate

                                                               

Eduard Buchner (1860-1917)1897 found fermentation inbroken yeast cells1907 Nobel Prize in Chemistry

The whole pathway in yeast and muscle cell were elucidated by

Arthur Harden1865-1940

Glycolysis• Glycolysis is an almost universal central pat

hway of glucose catabolism, the pathway with the largest flux of carbon in most cells.

• In some mammalian tissues (erythrocytes, renal medulla, brain, sperm), the glycolytic breakdown of glucose is the sole source of metabolic energy.

Glycolysis• Some of the starch-storing tissues, lik

e potato tubers, and some aquatic plants derive most of their energy from glycolysis.

• Many anaerobic microorganisms are entirely dependent on glycolysis.

1. phosphorylation of glucoseG 6-P

2. Isomerization of glucose 6-phosphate

G 6-P F 6-P

3. Phosphorylation of fructose 6-phosphate: the first committed step i

n glycolysis

F 6-P

F 1,6-BP

4. Cleavage of fructose 1,6-bisphosphate

F 1,6-BP

DHAP

G 3-P

5. Interconversion of the triose phosphate

6. Oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate

1,3-BPG

7. Phosphoryl transfer from 1,3-bisphosphoglycerate to ADP

3-PGA

Substrate-level phosphorylationsoluble enzymeschemical intermediates

Respiration-linked phosphorylationPhotophosphorylation

membrane-bound enzymestransmembrane gradients of protons

The formation of ATP by phosphoryl group transfer from a substrate is referred to as a subst

rate-level phosphorylation

Substrate-level phosphorylation

Respiration-linked phosphorylation or Photophosphorylation

H+ H+

ATP

ADP

Glyceraldehyde 3-phosphate dehydrogenase and Phosphoglycerate kinase are coupled in v

ivo• Glyceraldehyde 3-phosphate dehydro

genase catalyzes an endergonic reaction while phosphoglycerate kinase catalyzes an exergonic reaction.

• When these two reactions are coupled (which happens in vivo), the overall reaction is exergonic.

Glyceraldehyde 3-phosphate dehydrogenase

Phosphoglycerate kinase

G 3-P

Pi

NAD+

1,3-BPGNADH

ADP

3-PGAATP

8. Conversion of 3-phosphoglycerate to 2-

phosphoglycerate

2-PGA

The phosphoglycerate mutase reaction

The phosphoglycerate mutase reaction

3-phosphoglycerate2,3-bisphosphoglycerate2-phosphoglycerate

Phospho-glycerate mutase

COO- |HCOH | CH2O

9. Dehydration of 2-phosphoglycerate to phosphoenolpyruvate

PEP

10. Transfer of the phosphoryl group from phosphoenolpyruvate to ADP

Glucose + 2ATP + 2NAD+ + 4ADP + 2Pi

2 pyruvate + 2ADP + 2NADH + 2H+ + 4ATP + 2H2O

Glucose + 2ADP + 2NAD+ + 2Pi 2 pyruvate + 2ATP + 2NADH + 2H+

在有氧狀況下，產生的 NADH 很快就被送到 mitochondria 中用來合成 ATP

F 1,6-BP

DHAPG 3-Paldolase

pyruvateATPATPpyruvate

2-PGA

3-PGA

2-PGA

ATP3-PGANADH+ H+1,3-BPG

NAD+

Pi

F 1,6-BP

G 6-P

F 6-PPhosphohexoseisomerase

G 6-PADPATP

glucose

ADP

HexokinaseMg2+

Mg2+

F 6-P ATP

PFK-1 Mg2+

DHAP G 3-P

G 3-P 1,3-BPGG 3-P NADH+ H+

1,3-BPG

ADP

1,3-BPG

ADP

3-PGA

ATP

2-PGA

H2OPEPH2OPEP

PEPPEP

ADPADP

PiNAD+ 3-PGA

2-PGA

enolase

Phospho-Glyceratemutase

Pyruvatekinase

Phospho-glyceratekinaseTriose

Phosphateisomerase

Glyceraldehyde3-phosphatedehydrogenase Mg2+

Mg2+

Mg2+

NAD+ (nicotinamide adenine dinucleotide) is the active form of niacin

• Niacin is the common name for nicotinamide and nicotinic acid.

• Nicotinic acid is the common precursor for NAD+ and NADP+ biosynthesis in cytosol.

Niacin (Vitamin B3)

Functions of NAD+ and NADP+

• Both NAD+ and NADP+ are coenzymes for many dehydrogenases in cytosol and mitochondria

• NAD+ is involved in oxidoreduction reactions in oxidative pathways.

• NADP+ is involved mostly in reductive biosynthesis.

Weight loss, digestive disorders, dermatitis, dementia

Niacin deficiency: pellagra

Niacin deficiency

• Because niacin is present in most of the food and NAD+ can also be produced from tryptophan (60 grams of trptophan 1 gram of NAD+), so it is not often to observe niacin deficiency.

• However, niacin deficiency can still be observed in areas where maize is the main carbohydrate source because maize only contain niacytin, a bound unavailable form of niacin. Pre-treated maize with base will release the niacin from niacytin.

Niacin deficiency

• Areas where sorghum is the main carbohydrate source will also observe niacin deficiency if niacin uptake is not being watched carefully.

• Sorghum contains large amount of leucine, which will inhibit quinolinate phosphoribosyl transferase (QPRT), an enzyme involved in NAD+ biosynthesis from tryptophan.

• Vitamin B6 deficiency can also lead to niacin deficiency because pyridoxal phosphate is a coenzyme in NAD+ biosynthesis from tryptophan.

Feeder pathways for glycolysisp.535

Stored glycogen and starch are degraded by phosphorolysis

• Glycogen and starch can be mobilized for use by a phosphorolytic reaction catalyzed by glycogen/starch phosphorylase. This enzyme catalyze an attack by Pi on the (14) glycosidic linkage from the nonreducing end, generating glucose 1-phosphate and a polymer one glucose unit shorter.

p.535

p.536

Branch point (16) is removed by debranching enzyme

p.536

P

P P P P

P

P

Pphosphorylase

Transferase activity ofDebranching enzyme-1,6 glucosidase activity ofDebranching enzyme

Digestion of dietary polysaccharides

• Digestion begins in the mouth with salivary -amylase hydrolyze (attacking by water) the internal glycosidic linkages.

• Salivary -amylase is then inactivated by gastric juice; however pancreatic -amylase will take its place at small intestine.

• The products are maltose, maltotriose, and limit dextrins (fragments of amylopectin containing 16 branch points.

p.535

Endo (-amylase) and exo enzymes

Digestion of dietary disaccharides

• Disaccharides must be hydrolyzed to monosaccharides before entering cells.

• Dextrin + nH2O n D-glucose• Maltose + H2O 2 D-glucose• Lactose + H2O D-galactose + D-glucose• Sucrose + H2O D-fructose + D-glucose• Trehalose + H2O 2 D-glucose

dextrinasemaltase

lactase

sucrase

trehalase

p.535

Lactose intolerance

• Lactose intolerance is due to the disappearance after childhood of most or all of the lactase activity of the intestinal cells.

p.535-6

Lactose intolerance

• Undigested lactose will be converted to toxic products by bacteria in large intestine, causing abdominal cramps and diarrhea.

p.535-6

2 NADH4 ATP

ADPF 1,6-BPADPF 6-P

ATP

Fructose metabolism in muscle and kidney

Fructose hexokinaseMg2+

F 6-P

ATP

PFK-1

F 1,6-BP

Glycolysis

p.536

F 1-P

G 3-P

DHAP

ADP

DHAPglyceraldehydeFructose 1-phosphatealdolase

G 3-P

ATP

ADPF 1-P

ATP

Fructose metabolism in liver

Fructose fructokinase

glyceraldehydeTriose kinase

Mg2+

Mg2+

Triosephosphateisomerase

p.536

UDP-GlcNAD+

NADH

NADH G 1-PUDP-Gal

Gal 1-PADP

ATP

Galactose metabolism (p.536,537)

• Galactose is phosphorylated by galactokinase in the liver.

• Then galactose 1-phosphate is converted to glucose 1-phosphate by a series of reactions.

galactose galactokinaseMg2+

Gal 1-P

UDP-Glc

UDP-glucose:Galactose 1-Puridylyltransferase

UDP-Gal

NAD+

UDP-glucose4-epimerase

Epimer and epimerase (p. 241)

• Two sugars that differ only in the configuration around one carbon atom are called epimers.

• Enzymes that catalyze inversion of the configuration about an asymmetric carbon in a substrate having more than one center of asymmetry are called epimerases.

CHO |

HO-C-H

H-C-OH |

CH2OH

|HO-C-H |

H-C-OH |

CHO |

|HO-C-H |

H-C-OH |

CH2OH

H-C-OH

H-C-OH |

mannoseD- glucoseD-

1

2

3

4

5

6

1

2

3

4

5

6

D-Mannose is a C2-epimer of D-glucose

Galactosemia inability to metabolize galactose due to lack o

f1. UDP-glucose galactose 1-phosphate uridylyltransferase (classical galactosemia)2. UDP-glucose 4-epimerase3. Galactokinase

Among these, deficiency of either 1 or 2 is more severe (1 is the most severe).

p.537

Galactosemia• Deficiency of transfera

se (or epimerase) will result in poor growth, speech abnormality, mental deficiency, and (fatal) liver damage even when galactose is withheld from the diet.

p.537

Man 6-P

F 6-P

ADP

Mannose metabolism

mannose

ATP

HexokinaseMg2+

Man 6-P

Phosphomannoseisomerase

p.537

Fermentation

• Fermentation is referring to the process when no oxygen is consumed or no change in the concentration of NAD+ or NADH during energy extraction.

p.538

Fermentation

• Under hypoxic conditions, oxidative phosphorylation will be the first to stop. Then citric acid cycle will come to a halt due to inhibition effect from NADH. As a result, glycolysis will be the only metabolic pathway that is available to energy production during hypoxia.

n ATP

n ADP

FAD2 NAD+

2 ATP2 FADH26 NADH4 CO2

2 ADP

2 Acetyl-CoA

2 FAD

6 NAD+

2 CO22 ATP

2 ADP

2 pyruvate2 NADH

Fermentation

• However, the oxidation of glyceraldehyde 3-phosphate consumes NAD+ that will not be regenerated under hypoxic condition because oxidative phosphorylation is not available.

Glucose

2 NAD+

Glycolysis 2 acetyl-CoA2 NADH

2 NAD+

PDH

CitricAcidcycle

2 NADH 2 NADH

6 NADH2 FADH2

Oxidativephosphorylation

2 NAD+

2 lactate2 NAD+

2 NADH 2 pyruvate

2 NADH2 pyruvate2 ATP

The purpose of fermentation is to regenerate NAD+

• In order to continue regenerating NAD+, cells come up a strategy.

• During fermentation, NAD+ is regenerated during the reduction of pyruvate, the product of glycolysis.

glucose

2 NAD+

2 ADP

glycolysis

fermentation2 lactate2 NAD+

2 NADH 2 pyruvate

2 NADH2 pyruvate2 ATPglucose

2 NAD+

2 ADP

glycolysis

fermentation2 lactate2 NAD+

2 NADH 2 pyruvate

2 NADH2 pyruvate2 ATPglucose

2 ADP

glycolysis

fermentation

Lactate fermentationglycolysis

2ATP

Lactate is being recycled in liver (Cori cycle)

muscleliver

glucose

2 pyruvate

2 lactate2 lactate

2 pyruvate6 ATP

glucose

Carl and Gerty Cori, 1947 Nobel Prize in Physiology and Medicine

Lactate fermentation only happened in larger

animals• Most small vertebrates

and moderate size running animals have circulatory systems that can carry oxygen to their muscles fast enough to avoid having to use muscle glycogen anaerobically.

http://www.mountain-research.org/CV/coelacanth.jpg

http://www.anac.8m.net/Images/coelacanth.jpg

Deep sea fish (below 4,000 m or more) coelacanth uses anaerobic metabolism exclusively. The lactate produced is excreted directly. Some marine vertebrates can do ethanol fermentation.

Ethanol fermentation

• Yeast and other microorganisms ferment glucose to ethanol and CO2.

• Pyruvate is first decarboxylated by pyruvate decarboxylase, which is absent in vertebrate tissues and in other organisms that carry out lactic acid fermentation. Acetaldehyde is the product of this reaction.

Pyruvate decarboxylase• The decarboxylatio

n of pyruvate by pyruvate decarboxylase produces CO2, which is the reason why champagne is bubbling.

Thiamine pyrophosphate (TPP) is the coenzyme of pyruvate decarbox

ylase

• Thiamine pyrophosphate is derived from vitamin B1 (thiamine).

• Lack of vitamine B1 will lead to beriberi (edema, pain, paralysis, death; Singhalese “I cannot” Signifying the person is too ill to do anything.).

Alcohol dehydrogenase catalyze the second step of ethanol fermentati

on• Alcohol dehydroge

anse reduces acetaldehyde, producing NAD+ and ethanol.

• This enzyme is present in many organisms that metabolize ethanol, including human.

Fermentation has commercial values

• Bacteria like Lactobacillus bulgaricus (yogurt) and Propionibacterium freudenreichii (swiss cheese) ferments milk to produce lactic acid or propionic acid and CO2.

Dr. Chaim Weizmann1874-1952First President of IsraelFound butanol and acetonefermentation in Clostridium acetobutyricum

Industrial fermentation is done in huge close vats

• Fermentors are huge closed vats in which temperature and access to air are adjusted to favor the multiplication of the desired microorganism.

• Some even immobilize the cells in an inert support so no effort is required to separate microorganisms from products after fermentation is completed.