Overview of Glycolysis

• The Embden-Meyerhof (Warburg) Pathway ♪ Essentially all cells carry out glycolysis ♪ Cellular location: cytosol ♪ Ten reactions - same in all cells - but rates differ ♪ Two phases:

i) 1st phase- investing phase: glucose →F-1,6-2P →2G-3-P

ii) 2nd phase-harvesting phase: produces two pyruvates, ATPs and NADH

♪ Three possible fates for pyruvate

Overview of Glycolysis

GlycolysisGlycolysis

• A. Energy Investment Phase:A. Energy Investment Phase:

Glucose (6C)

Glyceraldehyde phosphate (2 - 3C) (G3P or GAP)

2 ATP - used 0 ATP - produced 0 NADH - produced

2ATP

2ADP + P

C-C-C-C-C-C

C-C-C C-C-C

⑤

GlycolysisGlycolysis

• B. Energy Yielding PhaseB. Energy Yielding Phase

Glyceraldehyde phosphate (2 - 3C) (G3P or GAP)

Pyruvate (2 - 3C) (PYR)

0 ATP - used 4 ATP - produced 2 NADH - produced

4ATP

4ADP + P

C-C-C C-C-C

C-C-C C-C-C

GAP GAP

(PYR) (PYR)

Glycolysis

Overview of Glycolysis

Don’t panic!Don’t panic!

• All you have to memorize are the total reAll you have to memorize are the total reaction, action, threethree rate-limiting reactions, rate-limiting reactions, threethree inhib inhibitors,itors, two two substrate-level phosphorylation steps substrate-level phosphorylation steps and major regulation mechanisms.and major regulation mechanisms.

You are not responsible for any structures

First Phase of Glycolysis

• The first reaction - phosphorylation of glucose Hexokinase or glucokinase This is a priming reaction - ATP is consumed he

re in order to get more later ATP makes the phosphorylation of glucose spo

ntaneous

Hexokinase 1st step in glycolysis; G large, negative

☻Hexokinase (and glucokinase) act to phosphorylate glucose and keep it in the cell

☻Km for glucose is 0.1 mM; cell has 4 mM glucose

• So hexokinase is normally active!

☻Glucokinase (Kmglucose = 10 mM) only turns on wh

en cell is rich in glucose ☻Hexokinase is regulated - allosterically inhibited

by (product) glucose-6-P

A good illustration for the induced A good illustration for the induced fit modelfit model

Induced fitInduced fit: : Binding of glucose to Hexokinase Binding of glucose to Hexokinase induces a large conformational changeinduces a large conformational change

☻This same change in conformation is observed for ALL kinases! It also accounts for the fact that water cannot be used for hydrolysis of ATP unless we fool the enzyme by using xylose instead of glucose.

Rx 2: PhosphoglucoisomeraseRx 2: Phosphoglucoisomerase

• Glucose-6-P to Fructose-6-P ☻Why does this reaction occur??

next step (phosphorylation at C-1) would be tough for hemiacetal -OH, but easy for primary -OH

isomerization activates C-3 for cleavage in aldolase reaction

☻Ene-diol intermediate in this reaction ☻A moonlighting protein-double as a kind of

nerve growth factor

Rx 3: Phosphofructokinase

• PFK is the committed step in glycolysis! The second priming reaction of glycolysis Committed step and large, -ΔG - means PFK is highly re

gulated

Rx 4: AldolaseRx 4: Aldolase

• C6 cleaves to 2 C3s (DHAP, Gly-3-P)

☺Animal aldolases are Class I aldolases

☺Class I aldolases form covalent Schiff base intermediate between substrate and active site lysine

☺Understand the evidence for Schiff base intermediate

1

2

3

4

5

6

Rx 5: Triose Phosphate IsomerRx 5: Triose Phosphate Isomerasease

• DHAP converted to Gly-3-P

☺An ene-diol mechanism (know it!)

☺Active site Glu acts as general base

☺Triose phosphate isomerase is a near-perfect enzyme

Glycolysis - Second Phase

• Metabolic energy produces 4 ATP Net ATP yield for glycolysis is two ATP Second phase involves two very high ener

gy phosphate intermediates • .

– 1,3 BPG – Phosphoenolpyruvate

Rx 6: Gly-3-DehydrogenaseRx 6: Gly-3-Dehydrogenase• Gly-3P is oxidized to 1,3-BPG ☻Energy yield from converting an aldehyde to a carboxylic a

cid is used to make 1,3-BPG and NADH ☻Mechanism involves covalent catalysis and a nicotinamide

coenzyme - know it ☻Arsenate (AsO4

3-) can substitute for HPO42- in this rxn beca

use  it is very similar in structure and reactivity but resulting 1-arseno-3-phosphoglycerate is unstable and will spontaneously hydrolyze to 3-phosphoglycerate Oxidation of substrate is uncoupled from ATP synthesis: arsenate is a potent poison! Glycolysis becomes futile - no usable energy is extracted from glucose

Rx 7: Phosphoglycerate KinaseRx 7: Phosphoglycerate Kinase

• ATP synthesis from a high-energy phosphate

☺This is referred to as "substrate-level phosphorylation"

☺2,3-BPG (for hemoglobin) is made by circumventing the PGK reaction

Substrate-Level PhosphorylationSubstrate-Level Phosphorylation

• ATP is formed when an enzyme transfers a phosphate groupphosphate group from a substrate to ADP.

Enzyme

Substrate

O-

C=O C-O- CH2

P P P Adenosine

ADP(PEP)Example:

PEP to PYR

P PP

ATP

O-

C=O C=O CH2

Product (Pyruvate)

Adenosine

Rx 8: Phosphoglycerate MutaseRx 8: Phosphoglycerate Mutase

• Phosphoryl group from C-3 to C-2

☺Rationale for this enzyme - repositions the phosphate to make PEP

☺Note the phospho-histidine intermediates!

☺Zelda Rose showed that a bit of 2,3-BPG is required to phosphorylate His

Rx 9: EnolaseRx 9: Enolase

• 2-P-Gly to PEP

☺Overall G is 1.8 kJ/mol

☺How can such a reaction create a PEP?

☺"Energy content" of 2-PG and PEP are similar

☺Enolase just rearranges to a form from which more energy can be released in hydrolysis

Rx 10: Pyruvate Kinase

• PEP to Pyruvate makes ATP

• These two ATP (from one glucose) can be viewed as the "payoff" of glycolysis

• Large, negative G - regulation!

The Fate of NADH and PyruvateAerobic or anaerobic??

• NADH is energy - two possible fates: – If O2 is available, NADH enters into Mitochondria by

two ways, where it is re-oxidized in the electron transport pathway, making ATP in oxidative phosphorylation.

– In anaerobic conditions, NADH is re-oxidized by lactate dehydrogenase (LDH), providing additional NAD+ for more glycolysis

Fates of PyruvateFates of Pyruvate

Go’= -146 kJ/mol

Potential energy = 2840 kJ/mol

Go’= -235 kJ/molGo’= -196 kJ/molGo’= -1160 kJ/mol

Energetics of GlycolysisEnergetics of Glycolysis• The elegant evidence of regulation!

Standard state G values are scattered: + and -

G in cells is revealing: – Most values near zero

– 3 of 10 Rxns have large, negative G Large negative G Rxns are sites of

regulation!

Other Substrates for GlycolysisOther Substrates for Glycolysis

• Glycerol, Fructose, mannose and galactose Gycerol is changed into DHAP Fructose and mannose are routed into glyc

olysis by fairly conventional means. Galactose is more interesting - the Leloir p

athway "converts" galactose to glucose - sort of....

Significance of GlycolysisSignificance of Glycolysis

Produce ATPs Provide biosynthetic materials Glycolysis and Cancer

• Hypoxia(组织缺氧 ) and hyoxia-inducible factor