What is an endergonic reaction?
A reaction that is not spontaneous ΔG > 0
What is an exergonic reaction?
A reaction that is spontaneous
ΔG < 0
How can an endergonic reaction be made spontaneous?
Couplling with an exergonic reaction that has a larger change in ΔG between reactants and products
What is one of the most common reactions used to drive thermodynamically unfavourable reactions?
Hydrolysis of ATP
What is the value of ΔG for the hydrolysis of ATP?
What is this value under cellular conditions?
-30.5KJmol-1
-50KJmol-1
What is ΔG°’?
The standard Gibbs free energy change under biological conditions
How is ΔG calculated from ΔG°’?
ΔG = ΔG°’ + RTln{Keq}
Why is the hydrolysis of ATP so exothermic?
1. Phosphate and ADP have more resonance stabilisation than ATP so the negative charge can be dissipated across more of the molecule thus stabilising the structure
2. Electrostatic repulsion. At pH 7 ATP has 4 negative charges which are in close proximity which weakens P-O-P bonds
3. Stabilisation due to hydration. More water can bind to ADP and Pi than ATP due to enthalpy and entropy considerations
What is the ATP turnover in humans during exercise?
0.5Kg/min
What is used to buffer ATP?
Phosphcreatine (PCr)
In which type of cell is PCr concentration high?
Myocytes
How is PCr concentration measured?
31P NMR spectroscopy
Give 3 examples of coupling ATP hydrolysis to chemical reactions in the cell
1. Phosphorylation of glucose to provide enough energy to prime the molecule to be broken down to pyruvate
2. Peptides are unstable thermodynamically (ΔG ≈ +17KJmol-1 for the formation of a dipeptide) but ATP can be used to build long peptide chains
3. Joining 2 nucleic acids at the start of DNA syntheis
What does biotin carry?
CO2
What is the carrier molecule of glucose?
Uridine diphosphate glucose
What does NAD stand for?
Nicotinamide adenine dinucleotide
What is the functional difference between NAD and NADP?
NAD = the main redox system for energy producing pathways
NADP = biosynthesis
How does the cell achieve 2 different redox potentials?
The phosphate group on NADP acts as a tag to allow recognition by biosynthetic enzymes.
Cell acheives redox potential for:
-production of ATP
-synthesis of other metabolites
What is the value of ΔG°’ for the hydrolysis of acetyl-CoA?
-31.5KJmol-1
What is the importance of the blood in metabolism?
Carries important metabolites such as glucose, fructose, lipoproteins, fatty acids, ketone bodies, amino acids and triacylglycerols
Define metabolite
A substance formed in or necessary for metabolism
What is the importance of the small intestine in metabolism?
Absorption of glucose, fructose and amino acids and transfer them into the blood
Fats are packed and transferred to the lymph and then the blood
What is the importance of the liver in metabolism?
Central role in glucose homeostasis
Synthesises and exports triglycerides to adipose tissue
Partially oxidises fats to produce ketone bodies
Nitrogen recycling and excretion/amino acid metabolism
Why is adipose tissue important in metabolism?
The major fat store and energy reserve of the body
What metabolite does the brain use to maintain neuronal cell function?
Primarily glucose but can use ketone bodies during fasting
Why is control necessary in metabolic pathways?
1. To avoid futile cycles
2. To link energy production to consumption
3. To respond to physiological changes
When may a futile cycle be possible?
When pathways run in opposite directions e.g. glycolysis ang gluconeogenesis at PFK1/fructose-1,6-bisPase
How can the amount of enzyme available for a reaction be changed?
Alterring rate of synthesis (often by regulating gene transcription)
Alterring rate of destruction
Give 3 examples of how enzyme concentration changes to meet physiological demands?
1. Increased lipoprotein lipase in lactating mammary glands
2. Changes in liver enzymes during the shift from the fed state to starvation
3. Increases in drug-metabolising enzymes following the intake of foreign compounds (and alcohol)
How is enzyme activity controlled?
Change in amount of enzyme
Metabolic control
Is metabolic control of an enzyme faster or slower than changing the concentration?
Faster
Give an example of metabolic control of an enzyme
Feedback inhibition
The product of a pathway turns off the steps at the start, thus preventing the accumulation of intermediates in that pathway if the product is present in high concentrations
What is the overall equation for aerobic respiration?
Glucose + Oxygen → Carbon dioxide + Water + ATP
What are erthyrocytes?
Red blood cells
Which tissues take up glucose in an insulin-independent manner?
-Brain
-Liver
-RBCs
Which glucose transporters are insulin independent?
GluT1, GluT2 and GluT3
Which tissues take up glucose in an insulin dependent manner?
Adipose and muscle
Which glucose transporter is glucose dependent?
GluT4
Why is uptake insulin dependent in some tissues?
Metabolism in these cells is controlled by hormonal signalling
How are GluT4 proteins moved from the inner cell to the membrane?
Initially GluT4 proteins are in vesicles which are recruited to the membrane by insulin
Where/when is anaerobic respiration necessary?
Cells which lack mitochondria e.g. RBCs and cells in the retina
In exercise
How is oxygen debt repaid?
Increasing the rate of the citric acid cycle to oxidise the lactate produced
What are the 2 phases of glycolysis?
1. Chemical priming phase
2. Energy yielding phase
What is the overal reaction of glycolysis?
Glucose → Pyruvate + ATP
What is the overal reaction for the citric acid cycle
Pyruvate + NAD+ + FAD → CO2 + NADH + FADH2
How many molecules of ATP are generated from 1 cycle of anaerobic glycolysis?
2
How many molecules of ATP are generated in each cycle of aerobic glycolysis?
29.5
What are the 2 fates of NADH produced in glycolysis?
1. Transported to mitochondria for oxidation
2. Regeneration of NAD+ by reducing pyruvate to lactate via lactate dehydrogenase
What are the clinical consequences of the dysregulation of glycolysis?
Neurodegenerative disease
Amplification of ischaemic damage
Proliferation of cancer
For glycolysis, what is the value of:
ΔG°’
ΔG
ΔG°’ = +23.9KJmol-1
ΔG = -1.3KJmol-1
Which reactions in glycolysis are controlled by allosteric regulation?
What is the common feature of these reactions?
Glucose → glucose-6-phosphate
Fructose-6-phosphate → fructose-1,6-bisphosphate
Phosphophenolpyruvate → pyruvate
Large change in ΔG
What happens to the aldehyde group in glyceraldehyde 3 phosphate?
Oxidised by NAD+ to carboxylic acid
C=O bond becomes energetically favourable -COOH
Where does the energy to phosphorylate ADP in the final reaction (using pyruvate kinase) come from?
Replacing a C=O and C=C bond with 2 C=O bonds
What are the 3 stages of glycolysis?
1. Lysis
2. Oxidation
3. Rearrangement
How much ATP, NAD and NADH is used/produced in the lysis stage of glycolyis?
2 ATP consumed
How much ATP, NAD and NADH is used/produced in the oxidation stage of glycolyis?
Produces 2ATP and 2NADH per molecule of glucose
How much ATP, NAD and NADH is used/produced in the rearrangement stage of glycolyis?
Produces 2 ATP per glucose
What is the net yield of glycolysis?
2ATP
2NADH
2 Pyruvate
What is the net reaction of glycolysis?
glucose + 2Pi + 2ADP + 2NAD+ → 2 pyruvate + 2ATP + 2NADH +2H+ + 2H2O
What happens to pyruvate in anaerobic respiration?
Converted to lactate by lactate dehydrogenase
What are the 2 fates of lactate produced in glycolysis?
1. Transported to the liver (cori cycle)
2. Converted back to pyruvate for oxidation of the carbon backbone in the citric acid cycle
What is the equation for the formation of lactate from pyruvate?
Pyruvate + NADH → lactate + NAD+
What is another term for fast twitch muscle?
White muscle
What is another term for slow twitch muscle?
Red muscle
Where does fast twitch muscle derive most of its energy from?
Anaerobic glycolysis
By how much can the rate of anaerobic glycolysis increase during exercise?
more than 1000 fold
How long do muscle glycogen stores last during bursts of intensive exercise?
80 seconds (but exhaustion after 20)
How long do phosphocreatine stores last during intensive exercise?
4 seconds
Which type of glycolysis would middle distance (400-800m) runners use?
Both aerobic and anaerobic
What is the normal concentration of lactate in the blood?
1mM
What is the pKa of lactate?
What does this mean for lactate in the blood?
3.86
It is fully dissociated
At what concentration of lactate does the buffering capacity of blood become overpowered?
What happens to blood pH when this occurs?
How is this caused?
5mM
drops to about 7
tissue hypoxia/decreased clearance of lactate from blood
Where does hexakinase act?
What is its Km?
Muscle
10mM
Where does glucokinase act?
What is its Km?
Liver
0.1mM
What is the advantage of the low Km of glucokinase?
The liver can deal with high concentrations of glucose
Prevents the liver from taking up the low conc of glucose it releases during fasting
What is the advantage of the high Km of hexakinase?
The muscle can operate at Vmax under low glucose conditions
What enzyme inhibits hexakinase?
glucose-6-phosphate
How does glucose-6-phosphate regulate glucose break down?
Glycolysis is shut off by build up of G-6-P because the enzyme inhibits hexakinase
This is important because the first step of glycolysis is irreversible as there is no G6Pase present in the muscle
Allows muscle to conserve glucose
(Does not inhibit glucokinase so glycogen and lipid synthesis can still occur in the liver)
What determines glucose uptake in the liver?
Blood glucose
What are the 3 fates of glucose-6-phosphate?
1. Glycolysis
2. Fatty acid synthesis via pentose phosphate pathway
3. Glycogen synthesis
Which molecules inhibit glycolysis via PFK1
What overcomes this inhibition?
Citrate and ATP
AMP (produced during exercise)
What relationship does ATP have to PFK1?
Both a substrate and allosteric inhibitor
Which enzyme catalyses the reaction?
2ADP ⇔ AMP + ATP
Adenylate kinase
How can the concentration of AMP be measured by considerring the reaction catalysed by adenylate kinase?
{AMP} = {ADP}2/{ATP}
Why is AMP a very sensitive indicator of energy status within the cell?
A small decrease in ATP results in a large increase in AMP
{AMP} is what % of {ATP}?
2%
How is control of PFK1 maintained in muscle?
AMP
increased AMP = increased glycolysis
How is control of PFK1 maintained in the liver?
Fructose-2,6-bisphosphate
increase F-2,6-BP = increased glycolysis and decreased gluconeogenesis
How is fructose-2,6-bisphosphate produced?
What does it activate?
Phosphorylation of fructose-6-phosphate by phosphfructokinase-2 (PFK-2)
PFK-1
Which molecule stimulates pyruvate kinase?
Why is this significant?
Fructose 1,6 phosphate
Feedforward stimuation: ensures the start of glycolysis stimulates the end of the pathway where ATP is generated
What has Eric Newsholme proposed is the importance of substrate cycles?
They serve an important regulatory purpose of signal amplification in tissues such as skeletal muscle:
At the cost of expending some ATP, the system is made more sensitive to changes in conc of regulatory molecules
(substrate cycling has also been linked to thermogenesis and treating obesity)
What are the 4 possible fates of pyruavte in organisms?
1. Acetyl CoA
2. Glucose
3. Lactate
4. Ethanol (fermentation in yeast)
What is the Warburg effect?
Tumours have a high glycolytic rate because running glycolysis faster promotes the pentose phosphate pathway (PPP)
PPP produces ribose for nucleotide synthesis, NADPH for fatty acid synthesis and glutathione reduction
These substrates are used for producing DNA and cell membranes
