24-01-22 - Glycolysis Flashcards
Learning outcomes
- Define the term glycolysis and know where in the cell it takes place
- Describe the major steps in the pathway and know where energy input is required, where energy is generated and the net yield of ATP
- Explain the role of nicotinamide adenine dinucleotide and the significance of lactic acid production
- Describe the key regulatory enzymes involved in the pathway and the different ways the rate of glycolysis is changed according to body requirements
What is glycolysis the first step in?
What is it a key pathway for?
Where does glycolysis take place?
How is blood glucose transported into cells?
What are the 3 potential fates of glucose within the cell?
- Glycolysis is the first step in the oxidation of glucose
- Glycolysis is a key pathway in preparing glucose (and other CHO) for oxidative degradation to generate ATP
- The enzymatic reactions of Glycolysis takes place freely in the cytosol
- Blood glucose is transporters into the cell via GLUT2 and GLUT 4 transporters, which lowers blood glucose concentrations
• Fate of glucose within the cell:
1) Glycolysis (catabolism)
2) Glycogen (anabolism)
3) Fat (anabolism)
What are the 2 roles of glycolysis?
What are the 3 phases of glycolysis?
What is the net yield of glycolysis per molecule of glucose?
How fast is glycolysis?
• 2 roles of glycolysis:
1) Generate ATP
2) Synthesise 2 pyruvates (3 carbon molecule) from one molecule of glucose (6 carbon molecule) which can then be fed into other pathways
• 3 phases of glycolysis:
1) Investment phase – 2 ATP is needed
2) Cleavage phase – cleaving 6 carbon glucose to 2 3 carbon intermediates
3) Energy harvest – converting ADP into ATP
- Glycolysis generates 4 ATP, but 2 ATP is invested in the investment phase, which gives a net gain of 2 ATP per molecule of glucose
- Glycolysis is a fast process and can generate a large amount of energy in a short period of time
Glycolysis overview
What are the first 2 steps of glycolysis?
• First 2 steps of glycolysis:
1) Phosphorylation of glucose at Carbon 6
• Requires 1 ATP (investment phase)
• Locks glucose inside the cell (maintains glucose gradient)
2) Conversion of Glucose-6-P (aldose) to Fructose–6–P (ketose)
• This helps raise the free energy content in the bonds
• G–6–P ring structure enables isomerisation and subsequent ring closure to F–6–P
What are steps 3,4, and 5 of glycolysis?
3) F–6–P phosphorylated at Carbon 1 to form Fructose-1,6-biphosphate (FBP)
• Requires 1 ATP (investment phase)
• Phosphofructokinase is a key regulatory point, as it is the first step unique to glycolysis
4) (also step 5) Aldolase cleaves FBP (6 carbons) into 2 trioses (carbons)
• Forms 2 Glyceraldehyde-3-Phosphate (GAP)
What are steps 6 and 7 of glycolysis?
• Steps 6 and 7 of glycolysis:
6) Oxidation and phosphorylation of GAP by NAD+ and Pi
• First high energy intermediate – aldehyde oxidation (exergonic reaction) drives synthesis of the 1,3 – biphosphoglycerate
• Aerobic conditions – 2 NADH + 2H+ enters the citric acid cycle
7) First formation of ATP (energy harvest)
• The newly formed high-energy phosphate bond used to synthesise ATP and 3-phosphoglycerate (3PG)
What are steps 8, 9, and 10 of glycolysis?
• Steps 8, 9 and 10 of glycolysis
8) 3PG converted to 2PG
• essential preparation for next energy harvest step
9) 2PG dehydration to form phosphoenolpyruvate (PEP)
• Converts low-energy phosphate ester bond of 2PG into high-energy intermediate phosphate bond
10) Hydrolysis of PEP high energy bond
• Generates ATP and pyruvate (physiological irreversible reaction
• Example of substrate-level phosphorylation, as ATP is generated from ATP and a phosphorylated intermediate, rather than ADP and Pi, like in oxidative phosphorylation
How much energy does glycolysis release form glucose?
Where is the majority of energy from glucose released?
What kind of phosphorylation takes place in glycolysis?
What are the 3 fates of pyruvate after glycolysis?
- Glycolysis releases only a small amount of free energy (2 net ATP)
- Most of the energy from glucose is released via the citric acid cycle and oxidative phosphorylation
- Glycolysis ATP formation is an example of substrate-level phosphorylation, as ATP is generated from ATP and a phosphorylated intermediate, rather than ADP and Pi, like in oxidative phosphorylation
• Fates of glucose after glycolysis:
1) Anaerobic cellular conditions – converted to lactate
2) Aerobic cellular conditions – converted to Acetyl CoA
3) High cellular energy levels – fatty acids of ketone bodies (after a big meal)
What is the fate of pyruvate in aerobic conditions?
What happens to NADH?
What occurs when cellular energy levels are in excess?
- In aerobic conditions, pyruvate is transported into the mitochondrial matrix and converted to acetyl CoA, which is then fed into the citric acid cycle
- NADH + H+ is oxidised to replenish NAD+ via oxidative phosphorylation, so it can be used for glycolysis again
- When cellular energy levels (ATP) are in excess, Acetyl CoA is used for fatty acid or ketone body synthesis
What needs to be done for glycolysis to continue in anaerobic conditions?
How is this done in anaerobic conditions?
What happens to pyruvate in anaerobic conditions?
What does this reaction allow?
What does the build-up of lactate cause in muscles?
What does lactate do when it is formed?
What determines how quickly a muscle fatigues?
- For glycolysis to continue in anaerobic conditions, NAD+ must be replenished
- Homolytic fermentation regenerates NAD+ when ATP demand is high and O2 is depleted
- In anaerobic conditions, the conversion of pyruvate to lactate with lactate dehydrogenase is favoured, which also replenishes NAD+
- This is a reversible reaction that enables glycolysis to continue for short amounts of time.
- Build-up of lactate in muscles causes muscles cramps and limits activity due to the increase in acidity
- When lactate is formed, it diffuses out of the cell, and returns to the liver, where is can be regenerated back into glucose
- Slow/fast twitch muscle fibres determine rates at which muscles fatigue
- Slow twitch fibres contain more mitochondria and O2 to ensure oxidative phosphorylation can continue
- Fast twitch muscle fibres, such as those found in sporadically used muscles like the thigh, evolve to make use for this system
- Their myosin can rapidly hydrolyse ATP
What is an isozyme?
What is the role of lactate dehydrogenase?
How many isozymes are there of lactate dehydrogenase?
What is the structure of lactate dehydrogenase like?
• Isozymes are different versions of the same enzyme, which catalyse the same reaction
• They may have different genes coded into them, or different amino acids
• Lactate dehydrogenase converts pyruvate to lactate, and in the process, reoxidises NADH to NAD+
• Lactate dehydrogenase has 5 different isozymes, each of which are tissue specific
• Lactate dehydrogenase has a tetrameric structure consisting of 4 subunits
• Each subunit is either a heart subunit or muscle subunit
• Each tissue specific isozyme has a different combination of H and M subunits, with there being 4 muscle subunits found in skeletal muscle versions of lactate dehydrogenase, and 4 heart subunits found in cardiomyocytes
• The heart subunits (LDHB gene – chromosomes 12) is aerobic and uses more oxidative phosphorylation
• The muscle subunit (LDHA gene – chromosome 11) is more anaerobic, and primarily uses glycolysis for energy
How can the different forms of lactate dehydrogenase be used in diagnosis?
- Different isozymes of lactate dehydrogenase will be elevated in the blood presence of different conditions
- LDH1 is elevated in myocardial infarction
- LDH5 is elevated in skeletal and liver diseases
What are the 6 various ways glycolysis is regulated?
• Glycolysis can be regulated by:
1) Key enzymes
• Hexokinase
• PFK
• Pyruvate Kinase
2) High ATP concentration leads to ATP feeding back and inhibiting enzymes
3) Intermediate substrates stimulate PFK activity (e.g fructose-6-P can stimulate or inhibit
4) High citric acid concentration inhibits – first intermediate in the citric acid cyle
5) Low pH inhibits – Build up of NADH and H+ ions
6) Hormones e.g insulin and glucagon
What are the 3 key enzymes in glycolysis?
What are 5 key regulation points in Glycolysis?
• Key enzymes:
1) Hexokinase
2) PFK
3) Pyruvate Kinase
• Regulation points in glycolysis:
1) Hexokinase allosterically inhibited by G-6-P
2) Phosphofructokinase
• Most important site of control;
• First step unique to glycolysis
3) High ATP inhibits PFK
• Allosterically binds to separate region from active site
4) High AMP activates PFK
• 2ADP to ATP + AMP
5) Pyruvate kinase inhibited by high ATP and alanine activated by FBP
