Chapt. 22 Glycolysis
Ch. 22 Glycolysis
Student 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
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
Fig. 1*
Anaerobic glycolysis
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
Fig. 2
Glycolysis phases
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
Fig. 3
Glycolysis step 1
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
Fig. 4
Glycolysis phase I
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
Fig. 5 top
Glycolysis phase II
Oxidation, substrate level phosphorylation yield
2 NADH, 4 ATP from 1 Glyceraldehyde 3-P
Key enzymes:
Glyceraldehyde 3-P dehydrogenase
• High-energy bond
Pyruvate kinase:
• Regulated step
Fig. 5 lower
**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 eto inner membrane FAD enzyme
• Loses some energy
• FAD(2H) not NADH
Bacteria not need shuttle
since only 1 compartment
Fig. Fig.
5 top7
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
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)
Fig. 10
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
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
Fig. 13
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
Lactic acidosis:
• Excess lactic acid in
blood > 5mM
• pH < 7.2
• From increased
NADH/NAD+
Many causes ->
• Excess alcohol
• Hypoxia
•
Fig. 15
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
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
Fig. 17