Fermentation + Pentose Phosphate Pathway

Core concept of the warburg effect

Cancer cells upregulate glycolysis (via fermentation of gluocose to lactate) even in the presence of oxygen.

Cancer cells prefers fermentation as a source of energy rather than oxidative phosphorylation.

Not just for ATP: provides intermediates for biosynthesis

Mitochondria is still functional but glycolysis supports rapid growth

How can glycolysis intervention be involved with cancer therapy?

compounds that inhibit key steps in glycolysis can kill cancer cells by limiting energy production

2-DG therapeutic targeting of cancer cell glycolysis

Mechanism: Competitive inhibitor of hexokinase (lacks C2-OH --> blocks step 2 glycolysis)

Limitations:

1. non specific effects all glucose dependent tissues

2. low potency requires more mM conc vs blood gluose (5-10mM)

PET scanning in tumor diagnosis

Radiolabeled analogs (e.g. F-FDG) detect tumors via PET scans

High glycolotic activity --> "glowing" metastes

Fates of pyruate in aerobic/anaerobic conditions

Aerobic conditions: pyruvate --> acetyl CoA (PDH complex) --> CAC --> full oxidation

Anaerobic conditions: fermentation. Regenerates NAD+ to sustain glycolysis

How are the fates of Pyruvate tissue specific?

Heart Muscle and Brain

- constant need for energy

- require full oxidation

- once pyruvate is made in cytosol (glycolysis) pyruvate travels to mitochondria

Skeletal Muscle

- upon exercise (anaerobic) muscles make lactate --> lactic acid --> secreted into blood --> liver

- convert pyruvate --> lactate via fermentation

Liver

- converts lactate --> glucose (via gluconeogenesis)

Key concept behind feeder pathways for glycolysis

Sugars enter glycolysis at the earliest possible step minimizing conversions. Their structure and stereochemistry determines the pathway

LDH composition and specificities/properties

tetramer (4 subunits--> 5 different combinations/isozymes) that comes in 2 types: M (muscle) LDH and H (hear) LDH

Isozymes have varying affinities for glucose/pyruvate and are tissue-specific

also have different gel electrophoresis patterns: M4 clustered on polar (-) end and H4 clustered on polar (+) end

H LDH

Heart LDH

Low pyruvate affinity/enzyme activity means pyruvate is mostly oxidized and excess substrate inhibition.

Prefers to convert lactate --> pyruvate to feed CAC (complete oxidation)

Forces aerobic, complete oxidation of glucose

M LDH

Muscle LDH with higher activity for bursts of energy by anaerobic glycolysis

found in muscle/liver

needs to rapidly convert pyruvate --> lactate during intense exercise

High pyruvate affinity ensures glycolysis keeps running even at low [pyruvate]

Favors fermentation

Anaerobic Glycolysis Basis

Fermentation

Generation of energy (pyruvate) without consuming NAD+

Reduction of pyruvate to another product

Regenerates NAD+ for further glycolysis under anerobic conditions

Lactic acid fermentation mechanism and physiology (lactate production)

Enzyme: LDH (Lactate Dehydrogenase)

Conversion of 2 pyruvate and 2 NADH --> 2 lactate and 2 NAD⁺

During strenuous exercise lactate builds up in muscle

Lactate can be transported to the liver and converted to glucose there

Oxidation-Reduction within Lactic acid fermentation and purpse

Oxidation of NADH --> NAD+ while reducing pyruvate to lactate

NAD+ can go back to glyceraldehyde free phosphate dehydrogenase and glycolysis proceeds

Ethanol fermentation Net reaction and mechanism

Glucose + 2ADP + 2PI --> Ethanol + 2CO2 + 2ATP

1st Step: Conversion of pyruvate to acetaldehyde and CO₂ by pyruvate decarboxylase

2nd Step: conversion of acetaldeyhde and NADH to ethanol and NAD⁺ by alcohol dehydrogenase

Main products of PPP and their purposes

NADPH (electron donor) and ribose-5-phosphate (precursor of nucleotides)

TPP

Thiamine pyrophosphate.

A common acetaldehyde carrier that forms a covalent bond with carbonyl carbon, forming an alcohol, and resulting in release of CO₂

PPP oxidative phase inputs, outputs, enzyme

Key enzyme: Glucose-6-Phosphate dehydrogenase (G6PD)

Inputs Glucose-6-phosphate

Outputs: ribose 5-phosphate and the production of 2 NADPH

Purpose of Non Oxidative phase in PPP

recylces pentoses into glycolitic intermediates when ribose-5-Phoshpate is not needed

PPP nonoxidative phase conversion

Conversion of ribulose-5-phosphate ---> glucose-6-phosphate Step 1: conversion to xylulose 5-phosphate

Step 2: transketolase and transaldolase reactions

Transketolases

Enzymes involved in nonoxidative phase of PPP

Uses TPP to transfer 2C units

Ketose is donor

Aldose is acceptor

Transaldolases

Enzymes involved in nonoxidative phase of PPP

Uses schiff base to transfer 3C units

Ketose is donor

Aldose is acceptor

Feedback control of PPP and glycolysis by NADPH and G6P

NADPH inhibits G6P (feeback control)

High NADPH --> GP6 enters glycolysis

Low NADPH --> G6P enters PPP (recycling)