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
*
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
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?
* 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
Describe lactate fermentation in humans and one other economically important lactate fermentation
* 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
Describe the mechanism of alcohol fermentation
Write the equations for the conversion of pyruvate to ethanol and for the overall glucose to ethanol reaction
1. Pyruvate (3C) is decarboxylated to two acetaldehyde (2C) molecules - this is catalysed by pyruvate decarboxylase
2. 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
What organisms are alcoholic fermentations associated with?
What are some uses of alcoholic fermentation?
* 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
Since fermentation is so inefficient at releasing energy from substrates why did organisms evolve to do it?
* 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
What is glycogen and where is it found?
* Glycogen is the major form of glucose storage in animals and human cells
* It is found mainly in the liver and skeletal muscle
31
When is glycogen synthesis carried out?
Outline the first stage of glycogen synthesis
* Animals synthesize and store glycogen when blood [glucose] is high
1. 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
Outline the second reaction in glycogen synthesis
* Pyrophosphate produced in the first step is rapidly hydrolysed by **pyrophosphatase**
* ****Pyrophosphate + H2O -\> 2 Pi
33
Outline the third reaction in the synthesis of glycogen
* Glycogen polymer is built around a protein core called **glycogenin**
1. First glucose molecule is joined covalently to glyocgenin
2. New glycosyl residues are added to the glycogen polymer by the enzyme **glycogen synthase**
1. This reaction involves the transfer of glycosyl units from UDP-glucose to the non-reducing ends of the glycogen strand
34
How is energy released from glycogen?
* 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
When glucose is no longer required for energy how is it converted into glycogen?
* 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
Why is the regulation of glycogen metabolism important?
* 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
What is the primary source of energy for muscle contraction?
* Glucose formed from the catabolism of glycogen is the primary source of energy for muscle contraction
38
What are the different forms of glycogen synthase?
* Dephosphorylated glycogen synthase I - **active**
* Phosphorylated glycogen synthase D - **inactive**
39
What is the main hormone responsible for the conversion of glucose to glycogen?
Where is it produced?
* 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
What are the main hormones responsible for the catabolism of glycogen?
* **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
How can glycogen be hydrolysed?
* 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
What are the different forms of glycogen phosphorylase?
How is it activated?
* 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
Why is cAMP referred to as a second messenger?
* cAMP is referred to as a second messenger since it is the intracellular agent of the hormone which is the first messenger
44
Describe the activation of the enzyme glycogen phosphorylase in the hydrolysis of glycogen
1. Hormonal stimulation (epinephrine or glucagon) of adenylyl cyclase is acheived by a conformational change in the hormone receptor
2. Hormone-receptor complex activates G-protein
3. G-protein activates adenylyl cyclase
4. Adenylyl cyclase converts ATP to cAMP
5. cAMP activates protein kinase A which phosporylates and activates the phosphorylase kinase enzyme
6. Phosphorylase kinase utilises ATP and activates glycogen phosphorylase
7. Glycogen phosphorylase stimulates glycogen hydrolysis
45
Describe the structure of G-proteins and how they are activated
* 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
1. In inactive state G-protein comlpex has GDP bound at the nucleotide site
2. Once stimulated by hormone-receptor complex GTP displaced GDP and binds to Gα
3. This binding interaction causes Gα to dissociate from Gβγ which in turn associates with adenylyl cyclase to activate adenylyl cyclase
4. Gα subunit uses its GTPase activity and hydrolyses GTP to GDP which reassociates with Gβγ to form inactive Gαβγ complex
46
What is the importance of GPCRs?
* 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
Give an overview of the regulation of glycogen phosphorylase
* Muscle glycogen phosphorylase is stimulated by AMP and inhibited by ATP and glucose-6-phosphate
48
Describe the interaction of inorganic phosphate with glycogen phosphorylase
* 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
Describe the interaction of ATP and glucose-6-phosphate with glycogen phosphorylase (GP)
* 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
Describe the interaction of AMP with the enzyme glycogen phosphorylase (GP)
* 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
Draw rate / concentration graphs for the effect of [Pi] on activity of glycogen phosphorylase including ATP, glucose-6-phosphate and AMP
* 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
