Chapt. 22 Glycolysis

Ch. 22 GlycolysisStudent Learning Outcomes:• Explain how glucose is universal fuel,

oxidized in every tissue to form ATP• Describe the major steps of glycolysis

• Explain decision point for pyruvate utilization depending on oxygen

• Describe major enzymes regulated

• Explain lactic acidemia and causes

Glycolysis overview

Fig. 1*Overview of Glycolysis, TCA cycle,

electron transport chain:• Starts 1 glucose phosphorylated

• 2 ATP to start process• Oxidation to 2 pyruvates

yields 2 NADH, 4 ATP• Aerobic conditions: pyruvate to TCA

cycle, complete oxidation• NADH from cytoplasm into mitochondria to ETC (waste some)• Complete oxidation total 30-32 ATP

Anaerobic glycolysis

Fig. 2

In absence of oxygen, anaerobic glycolysis:

• Recycles NADH to permit glycolysis continue

• Reduces pyruvate to lactate

• Only 2 ATP per glucose

• May cause lactic acidemia

Glycolysis phases

Fig. 3

Glycolysis phases:

• Preparation: • Glucose phosphorylated• Cleaved to 2 triose phosphates• Costs 2 ATP

• ATP-generating phase:• Triose phosphates oxidized

more• Produces 2 NADH• Produces 4 ATP

Glycolysis step 1

Fig. 4

1. Glucose is phosphorylated by Hexokinase with ATP:

• Commitment step• G6-P not cross plasma membrane

• Irreversible • Many pathway choices

• Glycogen synthesis needs G1-P

• Many tissue-specific isozymes of hexokinases

Glycolysis phase I

Fig. 5 top

2 ATP convert Glucose to Fructose 1,6 bis-P;• Fructose 1,6-bis-P split to 2 trioses• Glyceraldehyde 3-P (and DHAP isomerized)

Key enzymes:• Hexokinase

• PFK-1 • Commits to glycolysis• Regulated step

Glycolysis phase II

Fig. 5 lower

Oxidation, substrate level phosphorylation yield2 NADH, 4 ATP from 1 Glyceraldehyde 3-P

Key enzymes:Glyceraldehyde 3-P dehydrogenase• High-energy bond

Pyruvate kinase:• Regulated step

**Alternatie fates of pyruvate

Fate of pyruvate depends on availability of oxygen:• Much more ATP from complete oxidation of glucose• Aerobic: shuttles carry NADH into mitochondria; pyruvate

can be oxidized to Acetyl CoA and enter TCA• Anaerobic: pyruvate reduced by NADH to lactate, NAD+, H+

Fig. 6*

Aerobic: Glycerol 3-P shuttle carries NADH

Aerobic: Glycerol 3-P shuttle carries e- from NADH into mitochondrion; regenerates cytosol NAD+

• Glycerol 3-P diffuses across outer memberane, donates e-

to inner membrane FAD enzyme

• Loses some energy • FAD(2H) not NADH

Fig. 7Bacteria not need shuttle since only 1 compartment Fig. 5 top

Anaerobic:

Anaerobic glycolysis: NADH reduces pyruvate to lactate, regenerates NAD+ to continue glycolysis

1 glucose + 2 ADP + 2 Pi -> 2 lactate + 2 ATP + 2 H2O + 2 H+

• Lactate and H+ transported to blood; can have lactic acidosis

• Red blood cell, muscle, eye, other tissues• To maintain cell:

• Run faster• More enzymes• Use lot glucose

Fig. 9

Fate of lactate

Fig. 10

Fate of lactate:• Used to make glucose (liver) – Cori cycle• Reoxidized to pyruvate (liver, heart, skeletal muscle)

• lactate + NAD+ -> pyruvate + NADH• Lactate dehydrogenase (LDH) favors lactate, but if NADH used in

ETC (or gluconeogenesis), then other direction• Heart can use lactate -> pyruvate for energy• Isoforms of LDH: M4 muscle; H4 heart; mixed others)

II. Other functions of glycolysis

Glycolysis generates precursors for other paths:

• 5-C sugars for NTPs• Amino acids• Fatty acids, glycerol

• Liver is major site of

biosynthesis

Fig. 11

III. Glycolysis is regulated

Glycolysis is regulated by need for ATP:

• Hexokinase• Tissue specific isoforms• Inhibited by G-6-P • Except for liver

• PFK-1• Pyruvate kinase• Pyruvate dehydrogenase

• (PDH or PDC)

Fig. 12

Levels of ATP, ADP, AMP

Fig. 13

Levels of AMP in cytosol good indicator of rate ATP utilization

2 ADP <-> AMP + ATP

reaction of adenylate kinase

• Hydrolysis ATP -> ADP increases ADP, AMP

• ATP present highest conc:

• Small dec ATP -> large AMP

Regulation of PFK-1

Regulation of PFK-1:• Rate-limiting step, tetramer• 6 binding sites:

• 2 substrates: ATP, Fructose 6-P• 4 allosteric: inhibit ATP

• Activate by AMP• Activate by fructose 2,6 bis-P (product

when excess glucose in blood)

Fig. 14

Regulation of glycolysis enzymes

Regulation of pyruvate kinase:• R form (RBC), L (liver); M1/M2 muscle, others• Liver enz allosteric inhibition by compound in fasting;

• also inhibited by PO4 from Protein Kinase A

Regulation of PDH (PDC):• By PO4

to inactivate• Rate of ATP utilization• NADH/NAD+ ratio

Fig. 12

Lactic Acidemia

Fig. 15Lactic acidosis:

• Excess lactic acid in blood > 5mM

• pH < 7.2• From increasedNADH/NAD+

Many causes ->• Excess alcohol• Hypoxia•

Key concepts

• Glycolysis is universal pathway by which glucose is oxidized and cleaved to pyruvate

• Enzymes are in cytosol• Generates 2 molecules of ATP (substrate-level

phosphorylation) and 2 NADH• Pyruvate can enter mitochondria for complete

oxidation to CO2 in TCA + electron transport chain

• Anaerobic glycolysis reduces pyruvate to lactate, and recycles (wastes) NADH -> NAD+

• Key enzymes of glycolysis are regulated: hexokinase, PFK-1, pyruvate kinase, PDH C

Review question

Which of the following statements correctly describes an aspect of glycolysis?

a.ATP is formed by oxidative phosphorylation

b.Two molecules of ATP are used in the beginning of the pathway

c.Pyruvate kinase is the rate-limiting enzyme

d.One molecule of pyruvate and 3 olecules of CO2 are formed from the oxidation of 1 glucose

e.The reactions take place in the matrix of the mitochondria

Glyceraldehyde 3-P dehydrogenase

Fig. 17

Glyceraldehyde 3-P dehydrogenase uses covalent linkage of substrate to S of cys to form ~P:

• Covalent link to S of Cys; NAD+ nearby• Oxidation forms NADH + H+; ~S bond• NADH leaves, new NAD+

• Pi attacks thioester• Enzyme reforemd