Week 2 - glycolysis & glycogen metabolism Flashcards
1
Q
What is glycolysis?
A
- Glycolysis is the process that converts some of the metabolic energy of glucose molecules into ATP
2
Q
What are the phases of glycolysis?
Briefly explain what happens in both of them
A
- There are two phases of glycolysis
- In the first phase a series of five reaction convert glucose into two molecules of glyceraldehyde-3-phosphate
- This phase consumes two molecules of ATP
- In the second phase two molecules of glyceraldehyde-3-phosphate are converted into two molecules of pyruvate
- Results in the production of four ATP molecules
3
Q
What is the net production of ATP during glycolysis for one molecule of glucose?
A
- Net production is + 2 molecules of ATP per glucose molecule since two ATP are used up and four ATP are produced
4
Q
Outline, in detail, the first reaction of glycolysis
A
- Glucose is phosphorylated to glucose-6-phosphate by either hexokinase or glucokinase
- Occurs in the liver since glucokinase is specifically located here
- This reaction is thermodynamically unfavourable and requires the energy from the hydrolysis of ATP to drive the reaction forward
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5
Q
What is the importance of the first step of the first stage of glycolysis?
A
- The phosphorlyation of glucose to glucose-6-phosphate is important since the transport proteins that transport glucose into the cell cannot bind/transport glucose-6-phosphate meaning phosphorylated cannot leave the cell
- Furthermore the phosphorylation of glucose ensures that the intracellular concentration of glucose is low and the concentration gradient favours the diffusion of glucose into the cell
6
Q
Outline the second rection in the first stage of glycolysis
A
- The second step is the isomerisation of glucose-6-phosphate to fructose-6-phosphate
- This reaction is catalysed by the enzyme phosphoglucoisomerase (aka glucose phosphate isomerase)
7
Q
Outline the third reaction in the first stage of glycolysis
What is the importance of the enzyme that catayses this reaction?
A
- 3rd step is the phosphorylation of fructose-6-phosphate to fructose-1,6-biphosphate
- Reaction is catalysed by the enzyme phosphofructosekinase (PFK)
- Reaction is coupled with the hydrolysis of ATP which provides the phosphoryl group
- PFK is commonly described as the rate limiting enzyme in the regulation of the rate of glycolysis
8
Q
What step of glycolysis is the enzyme phosphofructokinase (PFK) involved in?
Why is it important and how is its activity controlled?
A
- PFK is involved in the second reaction of the first stage of glycolysis
- It is the rate limiting enzyme of glycolysis
- ATP is an allosteric inhibitor of PFK therefore high [ATP] inhibits PFK which decreases its affinity for fructose-6-phosphate and therefore decreases the rate of glycolysis
- Activity of PFK also depends on AMP which serves to revert the inhibition of ATP on PFK
- AMP levels increase as ATP levels decrease which alleviates the inhibitory action of ATP on PFK
9
Q
Outline the fourth reaction of the first stage of glycolysis
A
- Fructose-1,6-biphosphate is converted into two triose phosphates
- Glyceraldehyde-3-phosphate (G3P)
- Dihydroxyacetone phosphate (DHAP)
- This reaction is catalysed by fructose biphosphate aldolase
- G3P enters directly into the second phase of glycolysis
10
Q
Outline the fifth reaction of the first phase of glycolysis
A
- Dihydroxyacetonephosphate (DHAP) is converted to glyceraldehyde-3-phosphate via triose phosphate isomerase
- Once converted to G3P it enters the second phase of glycolysis
11
Q
Outline reaction 6 of glycolysis
A
- Reaction 6 marks the start of the second phase of glycolysis which, overall, results in the net production of ATP
- In reaction 6 glyceraldehyde-3-phosphate is oxidised to 1,3-biphosphoglycerate by the enzyme glyceraldehyde-3-phosphate dehydrogenase
- This reaction results in both the formation of carboxylic-phosphoric anhydride and the reduction of NAD+ to NADH
12
Q
Outline reaction 7 of glycolysis
A
- In this reaction 1,3-biphosphoglycerate is converted to 3-phosphoglycerate
- Phosphoglycerate kinase catalyses this reaction by catalysing the transfer of a phosphoryl group from 1,3-biphosphoglycerate to ADP to form ATP
13
Q
Outline reaction 8 of glycolysis
A
- Reaction is catalysed by phosphoglycerate mutase
- 3-phosphoglycerate is converted to 2-phosphoglycerate which involves moving the phosphoryl group from C-3 to C-2 by phosphoglycerate mutase
14
Q
Outline reaction 9 of glycolysis
A
- The enzyme enolase catalyses the formation of phosphoenolpyruvate (PEP) from 2-phosphoglycerate
- This reaction results in the formation of a water molecule
15
Q
Outline reaction 10 of glycolysis
A
- Pyruvate kinase mediates the transfer of a phosphoryl group from phosphoenolpyruvate (PEP) to ADP to form ATP and pyruvate
- This reaction requires Mg2+ and is stimulated by K+ and certain other monovalent ions
- This is the final ATP synthesizing reaction of glycolysis
16
Q
What are the main products of glycolysis?
How may they be processed?
A
- The main products are ATP, NADH and pyruvate
- Their processing depends on other cellular pathways
- NADH can be recycled to NAD+ by both aerobic and anaerobic pathways
- In aerobic conditions pyruvate enters the tricarboxylic acid cycle (TCA cycle) where it is oxidised to CO2 with the production of additional ATP, GTP, FADH2 and NADH
17
Q
Briefly describe what happens to pyruvate under anaerobic conditions in different organisms
A
- Pyruvate is reduced to ethanol in yeast
- In other microorganisms and animals pyruvate is reduced to lactate
- These anaerobic pathways are examples of fermentation
18
Q
Briefly decribe alcohol fermentation in yeast
Where is this used?
A
- Alcohol fermentation in yeast entails the decarboxylation of pyruvate to acetaldehyde by pyruvate decarboxylase
- Subsequently NADH reduces the acetaldehyde to ethanol which is catalysed by alcohol dehydrogenase
- The end products of this anaerobic pathways are CO2 and ethanol
- Used in the process of brewing beer and formation of wine
19
Q
Briefly outline lactic acid formation in animals
A
- Reduction of pyruvate to lactase occurs in oxygen-limited tissues, i.e. in tissues with limited access to blood flow and rapidly contracting skeletal muscle
- Pyruvte is reduced by lactase dehydrogenase to lactate when skeletal muscles are in anaerobic conditions as all oxygen has been consumed
- Pyruvate cannot be oxidised to enter the TCA cycle - lactate represents the end of glycolysis
20
Q
What can be the effects of the accumulation of lactic acid in animals?
How is the lactic acid further processed?
A
- Cramps and muscle fatigue is associated with the accumulation of lactic acid in the muscle
- Majority of lactate is transported out of muscle to the liver where it is used in gluconeogeneis for the re-synthesis of glucose
- Additionally, the production of lactate in skeletal muscles means there is a reduction in the amount of ATP generated and available because the majority is generated by the TCA cycle and oxidative phosphorylation
21
Q
What other substrates, besides glucose, can enter the glycolytic pathway?
A
- Other sugars can enter the glycolytic pathway providing that they can be converted into one of the intermediates of glycolysis
- These include:
- fructose
- mannose
- galactose
- glycerol
22
Q
Give an overview of anaerobic metabolism
A
- When no oxygen is present, pyruvate produced by glycolysis undergoes the process of fermentation
- This process consisits of reaction which reduce pyruvate to either lactate, an alcohol or an organic acid using the donated e- and H+ from NADH (NADH is oxidised) thus recycling NAD+ for use in glycolysis
- There is no additional energy gain during these reactions and the whole purpose if the fermentation process is to regenerate NAD+
- Anaerobic metabolism occurs in the cyctoplasm
- NADH + H+ ——–> NAD+ + 2e- + 2H+
23
Q
Describe the difference between the free energy of glucose and the energy released by the fermentation process
A
- Fermentation results in incomplete oxidation of glucose with a net generation of only 2 ATP molecules per glucose (produced during the glycolysis stage)
- Total free energy of glucose is 686kcal but the fermenation process releases only 2 ATP molecules (14.6kcal) per glucose molecule
- Therefore fermentation releases only 2.1% of the free energy of glucose and the majority of energy is conserved in the product of the fermentation reaction (lactate or alcohol)
24
Q
Describe different types of fermentation
A
- The most common type is lactate fermentation which is an energy yielding pathway found in anaerobic bacteria and animal cells (under anaerobic conditions)
- lactic acid is responsible for the taste of yogurt and sour milk
- Alcoholic fermentation commonly associated with yeasts cells and is economically important in food and drink production (break, wine, spirits)
- Propionate fermentation which gives distinctive cheese flavours and aromas
- Butylene glycol fermentation is associated with rancid fats
25
Q
Describe lactate fermentation
What is the equation for the conversion of pyruvate to lactate?
What is the overally equation for the conversion of glucose to lactate?
A
- Pyruvate (3C) is directly reduced to lactate (3C) using e- and H+ provided by NADH+H+; this regenerates NAD+
- This reaction is mediated by lactate dehydrogenase
- 2 x Pyruvate + 2NADH + H+ ——–> 2 x Lactate + 2NAD+
- The complete equation for the conversion of glucose to lactate@
- Glucose + 2ADP + 2P ——-> 2 lactate + 2 ATP + 2H2O
- Occurs in human cells during exercise when insufficient oxygen is taken in to generate enough energy to continue functioning during anaerobic exercise
- Build up of lactate in cells causes muscle cell malfunction, fatugue and cramps
- Anaerobic exercise can only take place over short period of time
26
Q
Describe lactate fermentation in humans and one other economically important lactate fermentation
A
- Humans
- Occurs in human cells during exercise when insufficient oxygen is taken in to generate enough energy to continue functioning during anaerobic exercise
- Build up of lactate in cells causes muscle cell malfunction, fatugue and cramps
- Anaerobic exercise can only take place over short period of time
- Economically, bacterial lactate fermentations are used in the production of fermented milk product such as cheese and yogurts
27
Q
Describe the mechanism of alcohol fermentation
Write the equations for the conversion of pyruvate to ethanol and for the overall glucose to ethanol reaction
A
- Pyruvate (3C) is decarboxylated to two acetaldehyde (2C) molecules - this is catalysed by pyruvate decarboxylase
- Two acetaldehyde molecules (2C) are reduced to two ethanol (2C) molecules by alcohol dehydrogenaase
- Odixation of NADH + H+ provides the e- and H+ for this reaction and results in the regeneration of NAD+
- Pyruvate to ethanol:
- 2 x Pyruvate + 2NADH + H+ ——–> 2 x Ethanol + 2NAD+ + 2CO2
- Glucose to ethanol:
- Glucose + 2ADP + 2Pi ——-> 2 ethanol + 2 ATP + 2H2O + 2CO2
28
Q
What organisms are alcoholic fermentations associated with?
What are some uses of alcoholic fermentation?
A
- They are associated with yeast and plant cells
- Break production - (ethanol fermentation) where the CO2 released causes the dough to rise - ethanol is evaportated during baking
- Wine production - (ethanol fermentation) where it provides the alcohol content and the CO2 provides the carbonation, however this is now usually done artificially
29
Q
Since fermentation is so inefficient at releasing energy from substrates why did organisms evolve to do it?
A
- The first cells formed 3.5 million years ago and the environment was anaerobic
- Although there was plenty of organic material oxygen only became present 2.5 million years ago
- These cells developed pathways for the breakdown of organic material and the production of ATP (glycolysis) under anaerobic conditions
- This cycle could only be sustained if NAD+ was recycled leading fermentation to develop
- The first cells used glycolysis and fermentation
- When O2 appeared on Earth cells built on existing processes to develop cellular respiration as an extentsion to cater for aerobic conditions which is a more efficient means of releasing energy from organic molecules
- The fermentation pathway is maintained in cells since it allows them to cope with occasional anaerobic conditions
30
Q
What is glycogen and where is it found?
A
- Glycogen is the major form of glucose storage in animals and human cells
- It is found mainly in the liver and skeletal muscle
31
Q
When is glycogen synthesis carried out?
Outline the first stage of glycogen synthesis
A
- Animals synthesize and store glycogen when blood [glucose] is high
- Synthesis of UDP-glucose which carries activated glucose molecules (in the same ways acetyl-CoA carries activated acetyl groups)
- Catalysed by UDP-glucose pyrophosphorylase
- Glucose-1-phosphate + UTP -> UDP-glucose + pyrophosphate
32
Q
Outline the second reaction in glycogen synthesis
A
- Pyrophosphate produced in the first step is rapidly hydrolysed by pyrophosphatase
- Pyrophosphate + H2O -> 2 Pi
33
Q
Outline the third reaction in the synthesis of glycogen
A
- Glycogen polymer is built around a protein core called glycogenin
- First glucose molecule is joined covalently to glyocgenin
- New glycosyl residues are added to the glycogen polymer by the enzyme glycogen synthase
- This reaction involves the transfer of glycosyl units from UDP-glucose to the non-reducing ends of the glycogen strand
34
Q
How is energy released from glycogen?
A
- Glycogen phosphorylase, present in both liver and muscle cells, phosphorylates glycogen to product glucose-1-phosphate monomers and a glycogen polymer shortened by one
- Glucose-6-phosphate is needed to proceed into glycolysis and the enzyme phosphoglucomutase converts glucose-1-phosphate into glucose-6-phoshate
- Phosphoglucomutase catalyses this reaction reversibly
35
Q
When glucose is no longer required for energy how is it converted into glycogen?
A
- When glucose is no longer required, phosphoglucomutase converts glucose-6-phosphate to glucose-1-phosphate which is then used in the production of UDP-glucose and glyogen production
36
Q
Why is the regulation of glycogen metabolism important?
A
- Glycogen is an energy reservoir and therefore the synthesis and degradation must be strictly controlled
- Glycogen metabolism is controlled by the reciprocal regulation of glycogen phosphorylase and glycogen synthase
37
Q
What is the primary source of energy for muscle contraction?
A
- Glucose formed from the catabolism of glycogen is the primary source of energy for muscle contraction
38
Q
What are the different forms of glycogen synthase?
A
- Dephosphorylated glycogen synthase I - active
- Phosphorylated glycogen synthase D - inactive
39
Q
What is the main hormone responsible for the conversion of glucose to glycogen?
Where is it produced?
A
- Insulin is the principle hormone responsible for the conversion of glucose to glycogen via an intracellular cascade which stimulates glycogen synthase
- Insulin is secreted by β cells of the islets of Langerhans in response to increased blood [glucose] and its role is to stimulate the removal and storage of glucose in the form of glycogen
40
Q
What are the main hormones responsible for the catabolism of glycogen?
A
- Epinephrine and glucagon stimulate the catabolism of glycogen
- When blood [glucose] decreases glucagon is secreted from the α cells of the islets of Langerhans
- Glucagon travels through the blood and binds to specific receptors on liver cell plasma membranes
- Signals released from the CNS stimulate the release of epinephrine (adrenalin) from the adrenal glands into the blood
- Epinephrine targets liver and muscle cells
- Both hormones bind to receptors on plasma membrane which stimulate an intracellular cascade that activates glycogen phosphorylase and inhibits glycogen synthase
41
Q
How can glycogen be hydrolysed?
A
- By both α- and β-amylases to produce maltose and glucose
- α-amylase is an important component of saliva and pancreatic juice whereas β-amylase is found in plants
- It can also be hydrolysed using glycogen phosphorylase, present in liver and muscle cells, to produce glucose-1-phosphate and a glycogen polymer shortened by one glucose residue
42
Q
What are the different forms of glycogen phosphorylase?
How is it activated?
A
- Similarly to glycogen synthase, glycogen phosphorylase exists in two forms:
- phosphorylated phosphorylase a - active
- dephosphorylated phosphorylase b - inactive
- Phosphorylation reaction that activates the enzyme is mediated y an enzyme cascade
- This cascade is initiated by hormonal stimulation of adenylyl cyclase which is a membrane bound enzyme that stimulates the conversion of ATP to cAMP
43
Q
Why is cAMP referred to as a second messenger?
A
- cAMP is referred to as a second messenger since it is the intracellular agent of the hormone which is the first messenger
44
Q
Describe the activation of the enzyme glycogen phosphorylase in the hydrolysis of glycogen
A
- Hormonal stimulation (epinephrine or glucagon) of adenylyl cyclase is acheived by a conformational change in the hormone receptor
- Hormone-receptor complex activates G-protein
- G-protein activates adenylyl cyclase
- Adenylyl cyclase converts ATP to cAMP
- cAMP activates protein kinase A which phosporylates and activates the phosphorylase kinase enzyme
- Phosphorylase kinase utilises ATP and activates glycogen phosphorylase
- Glycogen phosphorylase stimulates glycogen hydrolysis
45
Q
Describe the structure of G-proteins and how they are activated
A
- G-protein = GTP-binding protein
- G-proteins have a heterotrimeric structure consisting of α, β and γ subunits
- α-subunit binds to GDP or GTP and possess slow GTPase activity
- In inactive state G-protein comlpex has GDP bound at the nucleotide site
- Once stimulated by hormone-receptor complex GTP displaced GDP and binds to Gα
- This binding interaction causes Gα to dissociate from Gβγ which in turn associates with adenylyl cyclase to activate adenylyl cyclase
- Gα subunit uses its GTPase activity and hydrolyses GTP to GDP which reassociates with Gβγ to form inactive Gαβγ complex
46
Q
What is the importance of GPCRs?
A
- A single hormone-receptor complex can activate several G-proteins before the hormone dissociates from its receptor, therefore the enzyme cascade amplifies the hormone signal
47
Q
Give an overview of the regulation of glycogen phosphorylase
A
- Muscle glycogen phosphorylase is stimulated by AMP and inhibited by ATP and glucose-6-phosphate
48
Q
Describe the interaction of inorganic phosphate with glycogen phosphorylase
A
- Inorganic phosphate (Pi) is a substrate of GP and the binding is highly cooperative - this allows the enzyme activity to increase over a narrow range of [substrate]
- However, when the [ATP] or [glucose-6-phosphate] is low the activity of GP is regulated by the availability of its substrate Pi
49
Q
Describe the interaction of ATP and glucose-6-phosphate with glycogen phosphorylase (GP)
A
- Catabolism of glycogen is regulated by negative feedback loop inhibition of GP by ATP and glucose-6-phosphate
- They both have negative effect on susbtrate binding by decreasing the affinity of GP for Pi
- Therefore they serve as negative heterotropic effectors
- Means that GP is inhibited (therefore glycogen catabolism) at high [ATP] and [glucose-6-phosphate]
- However, when the [ATP] or [glucose-6-phosphate] is low the activity of GP is regulated by the availability of its substrate Pi
50
Q
Describe the interaction of AMP with the enzyme glycogen phosphorylase (GP)
A
- AMP binds to the same site as ATP on GP meaning that ATP and AMP are competitive
- AMP stimulates GP and therefore serves as a postivie heterotropic effector
- AMP enhances the affinity of GP for its substrate Pi
51
Q
Draw rate / concentration graphs for the effect of [Pi] on activity of glycogen phosphorylase including ATP, glucose-6-phosphate and AMP
A
- The substrate saturation curve is shifted to the right in the presence of ATP or glucose-6-phosphate
- Means that a higher substrate concentration is need to reach half-maximal velocity (Vmax/2)
- In the presence of AMP, the substrate saturation curve shifts to the left, which means that a lower substrate concentration is needed to achieve half-maximal velocity
- However, neither ATP or AMP causes the Vmax to change:
- km (Michaelis-Menten constant) is the concentration of substrate at which half-maximal rate is observed
