Metabolism And Control Flashcards
How is energy metabolised
Via
Glycolysis
TCA
Electron transport chain & oxidative phosphorylation
What are catabolic pathways
break down complex molecules into
simple molecules and release energy
What are anabolic pathways
build complex molecules from simple molecules and require energy usually in form of ATP
What metabolic pathways cytosolic
Glycolysis, PPP & fatty acid synthesis
Where are most enzymes for TCA cycle
located in the mitochondrial matrix, except succinate dehydrogenase, which is linked to the respiratory chain in the inner membrane
What are perxisomes
a small organelle present in the cytoplasm of many cells, which contains the reducing enzyme catalase and usually some oxidases
What are types of metabolic reactions
❖ Oxidation: loss of electrons
❖ Reduction: acquisition of electrons
❖ Usually coupled in a reaction where electrons are transferred from molecule to another
❖ hydrolysis - dehydration: add/remove water
❖ (de)phosphorylation: removal/addition of a phosphate group
❖ (de)carboxylation: removal/addition of a CO2 molecule
❖ Ligation reactions
An example of hydrolysis
hydrolysis - dehydration: add/remove water
Purpose of energy metabolism
❖ Primary purpose of energy metabolism is to provide a
constant supply of ATP to maintain cell growth
How can ATP be produced
either through substrate-level phosphorylation, a process which doesn’t require oxygen
or through oxidative phosphorylation, which uses oxygen
What bonds do ATP have
ATP molecule stores energy in 2 phosphoanhydride bonds
Hydrolysis of ATP releases
Inorganic phosphate
And (at physiological pH) releases 7.3 kcal as energy
4 different steps for ATP production
❖ Glycolysis (1 glucose —> 2 pyruvate)
❖ Oxidative decarboxylation of pyruvate to form acetyl-CoA (one per pyruvate) loss of one CO2 per pyruvate
❖ TCA cycle <— introduction of 2 carbon atoms in the form of acetyl-CoA, subsequent loss of 2 CO2 per acetyl- CoA
❖ Electron transport chain: energy which was stored in the form of energy rich H- (e.g. NADH) is converted to water (H2O) and ATP
Equation for ATP production
❖ C6H12O6 + 6O2 —> 6CO2 + 6H2O + 31ATP
What is the overall yield of glycolysis
Glucose = 2 x Pyruvate
2ADP = 2ATP
2NAD+ = 2NADH
Pathway of glycolysis
❖ Ordered series of enzymatic reactions in the cytosol
❖ Glucose is primed with two phosphorylation steps (energy investment) and one isomerisation to form F-1,6-bp
❖ F-1,6-bp is split to form GA-3-P
❖ GA-3-P is converted to Pyruvate
What can lactate cause
Exercising skeletal muscle
Increase in lactate
Muscle pain
Coronary arteries blocked by artherosclerosis
Insufficient O2 supply for heart
Increase Lactate
Chest pain (Angina pectoris)
Anaerobic respiration
NAD+ must be regenerated for glycolysis to continue, so lactate is formed
What is the cori cycle
Liver provides glucose for tissue glycolysis. The lactate produced is used by the liver to make glucose.
Lactate converted to Pyruvate then glucose 6 phosphate in the liver
Pathway of gluconeogensis
Synthesis of glucose from pyruvate
Some reactions in common with glycolysis
‘Irreversible’ steps use different enzymes
Costs energy:
2 Pyruvate + 4ATP + 2GTP + 2NADH makes one glucose
(Glucose makes 2 Pyruvate +2ATP + 2NADH)
What is pyruvate decarboxylation
❖ This is mediated by a large enzyme complex (pyruvate dehydrogenase) that converts pyruvate to acetyl-CoA
❖ Occurs within the mitochondria
❖ NAD+ is reduced to NADH
❖ One CO2 is produced (note: this is the first carbon which is lost from glucose in the process of converting glucose to CO2, H2O and energy)
What is the PDH complex (link reaction)
catalyzes the oxidative decarboxylation of pyruvate with the formation of acetyl-CoA, CO2 and NADH
increases the influx of acetyl-coA from glycolysis into the TCA cycle
PDH complex regulation
The pyruvate dehydrogenase complex is regulated by covalent modification through the action of a specific kinase and phosphatase; the kinase and phosphatase are regulated by changes in NADH, acetyl-CoA, pyruvate, and insulin.
What is carbon flux
???
How does pyruvate enter the TCA cycle
To enter the TCA cycle, pyruvate is used
to make Acetyl CoA
What process does gluconeogensis reverse
Glycolysis
Turns pyruvate into glucose
In the liver
Which steps in glycolysis are irreversible
Steps 1, 3, 10
What enzyme regulates glycolysis
Phosphofructokinase
What enzyme regulates gluconeogenesis
1,6, bisphosphatase
What happens when ATP is low
❖ When ATP is low phosphofructokinase and glycolysis are switched on to generate ATP
What happens when ATP is abundant
❖ When ATP is abundant phosphofructokinase is switched off and 1,6, biphosphatase is switched on driving ATP through gluconeogenesis to glucose
What happens when oxygen levels are low
❖ Pyruvate is converted to lactate when oxygen levels are low. Produces 2 ATP rapidly but much more is produced by TCA cycle. (Anaerobic respiration)
Lactate produced to regenerate NAD+
NADH oxidised to NAD
What is the Cory cycle
❖ Lactate from skeletal muscle is taken up by the liver to go through gluconeogenesis (Cory cycle)
What is the cyclic acid cycle
❖ Also known as the Tricarboxylic acid cycle
❖ Accounts for 2/3 of total oxidation of carbon
in most cells
❖ Takes place in the mitochondrial matrix
❖ Oxygen is required for the downstream electron transport chain as O2 is a final acceptor for NADH to lose electrons and NAD returns to the cycle
What does TCA cycle breakdown
❖ The TCA cycle is involved in the breakdown of all three major food groups (carbohydrates, proteins and lipids)
Mechanism of TCA cycle
❖ Through a series of reaction oxidise pyruvate to CO2
❖ Each cycle adds 2 carbon atoms as acetyl group and releases them in the form of CO2
❖ However, the carbons lost originate from oxaloacetate, not acetyl-CoA
❖ The energy of acetyl-CoA is stored in NADH and FADH2
Overall products of TCA cycle
Acetyl CoA - CoA and 2 CO2
3NAD+ - 3NADH
FAD - FADH2
GDP + Pi - GTP
Where is NAD derived from
NAD+ is derived from the vitamin niacin (B3)
What is NAD
❖ It acts as a coenzyme in several redox reactions
❖ It’s oxidation in the respiratory chain generates 2.5 molecules of ATP
Where is FAD derived from
vitamin riboflavin (B2)
What is FAD
❖ FAD attaches covalently to it’s enzyme (prosthetic group)
❖ Succinate dehydrogenase contains FAD and is bound to the inner membrane of the mitochondria and is an integral part of the respiratory chain
❖ FAD’s oxidation in succinate dehydrogenase generates 1.5 molecules of ATP
What kind of reaction is TCA cycle
Anaplerotic reaction
“fill in” missing metabolites for important metabolic pathways
Examples of Anaplerotic reactions
❖ Direct conversion of pyruvate to oxaloacetate (PC reaction)
❖ Oxaloacetate/aspartate conversion
❖ Glutamate/a-ketoglutarate
conversion
❖ Malate to pyruvate conversion (malic enzyme)
How many coenzymes are produced at the end of TCA cycle
10 NADH and 2 FADH2 molecules
Where are enzymes for ETC located
located in the inner mitochondrial membran
How much ATP does each coenzyme produce
Each NADH produces indirectly approx. 2.5 ATPs, whereas FADH2 will produce approx. 1.5 ATPs (lower energy content of the electrons compared to NADH)
What happens when NADH gets oxidised
loses a proton and 2 electrons:
❖ NADH —>NAD+ +2H+ +2e-
What is the last step in the ETC
2e- +2H+ +1/2O2 —>H2O
What happens when electrons leave NADH
❖ When the electrons leave NADH they are high in energy
❖ Each time they are passed on to one of the complexes, they lose energy
❖ This energy is used to pump protons from the mitochondrial membrane into the inter membrane space
❖ The proton gradient is then used to produce ATP
Where are the electrons transported
❖ the transport of 2 electrons through complex I and III will extrude 4 H+ each into the inter membrane space
What is set up when protons move across the intermembrane of the mitochondria
Electrochemical potential of 150-250mV and possible concentration gradient
What does the potential difference provide
provides the energy for ATP synthesis
❖ 3 H+ ions are needed to make 1 ATP (plus 1 H+ to translocate the ATP to the cytosol)
What is cytochrome c oxidase
catalyses the transfer of the electrons to molecular oxygen
can be inhibited by cyanide, carbon monoxide and azide
What is substrate level phosphorylation
transfer of phosphate from a substrate to ATP (GTP)
What is oxidative phosphorylation
formation of ATP coupled to oxidation of NADH or FADH2 by O2
How is energy converted to ATP
Energy from electron transport drives efflux of H+ from mitochondrial matrix
Proton electrochemical gradient formed
Proton gradient drives ATP synthase
Chemi osmotic coupling hypothesis
What is ATP synthase
The electrochemical gradient is used to drive synthesis of ATP via conformational change in ATP synthase
Location of NADH molecules
formed during glycolysis and located in the cytosol
Where is NADH oxidised
NADH can only be oxidised inside the mitochondria and NADH are unable to cross the mitochondrial membrane
What are the 2 mechanisms which enable ‘reducing equivalents’ (NADH) to be transferred from the cytosol into the mitochondrion
- Glycerol phosphate shuttle
- Malate/aspartate shuttle
What is the glycerol phosphate shuttle
(important in insects) uses cytosolic NADH to reduce DHAP to form glycerol-3-phosphate which then diffuses into the mitochondria and is oxidised by the mitochondrial glycerol-3- phosphate dehydrogenase to form DHAP and FADH2
What is the malate/ aspartate shuttle
❖ starts with cytosolic oxaloacetate
❖ malate dehydrogenase reduces OAA to form malate, which is then transported into the mitochondria
❖ inside the mitochondria the reaction is reversed by mitochondrial malate dehydrogenase
❖ Then to transport OAA back to cytosol it needs to be able to cross the inner mitochondrial membrane
❖ it has to be transaminated to aspartate which then can be transported into the cytosol, where it is converted back into OAA by the cytosolic aspartate aminotransferase
What is lost in the malate/ aspartate shuttle
while usually 10 H+ per NADH can be pumped across the membrane to form ATP, in this case the glutamate aspartate carrier (to maintain glutamate and aspartate concentrations) uses 1 H+. Hence the ATP production is only 2.25 molecules of ATP per cytosolic NADH
What is the ATP yield for insects
❖ Insects use the glycerol phosphate shuttle, where 2 cytosolic NADH yield 2 mitochondrial FADH2, hence the yield in insects would be 36 ATP
What is the yield for ATP for humans
Using the modern non-integer P/O ratios yield a total of 31 ATP for eukaryotes using the malate/aspartate shuttle and 29.5 ATP using the glycerol phosphate shuttle
Why is brown adipose tissue important
❖ Mitochondria largely uncoupled, so energy released as heat rather than captured as ATP
❖ Important for maintaining body temp, especially in neonates (hibernation)
What are UCP
Uncoupling proteins
Provide proton channel
Dinitrophenol is also an uncoupler
How is metabolism controlled
Primary control - the level of ATP
❖ Levels of intermediates affect local rates
❖ 3 major control strategies:
❖ Enzyme levels
❖ Enzyme activities
❖ Substrate availability
❖ Many points of control, mostly at steps unique to the pathway
What are the three control points of TCA cycle
Enzymes:
Pyruvate dehydrogenase
isocitrate dehydrogenase
𝛼 - ketoglutarate dehydrogenase
are all control points are they are inhibited by ATP, NADH and acetyl CoA
What are the different metabolic profiles of different organs
❖ Brain - consumes ~60% of body glucose at rest. Can use ketone bodies in starvation
❖ Muscle - resting muscle uses fatty acids. Anaerobic muscle draws on glycogen stores (75% of glycogen stored in muscle)
❖ Kidneys - 0.5% of body mass, use 10% of body glucose (Na+- K+ ATPase)
❖ Liver - major site of conversion (Cori cycle)
What diseases are associated with defects in carbohydrate metabolism
❖ A range of diseases resulting from mitochondrial defects (neuro/visual symptoms) - eg Leber hereditary optic neuropathy (complex I)
❖ Beriberi (VitB1, Pyr DeH, a-ketoglut DeH), mercury & arsenic poisoning (Pyr DeH and GAP -> 3PGA conversion)
❖ Diabetes, glucosuria
❖ Glycogen storage disease (eg von Gierke’s - absence of glucose 6- phosphatase; McArdle’s - myophosphorylase deficiency; Tarui - PFK; Cori; others)
❖ Cancer (Metabolomics; PKM2)
How do atp levels control insulin secretion
Insulin controls carbohydrate metabolism
Imbalance between insulin and glucagon
If theres high glucose or excessive ketone body production it stimulates insulin
The Warburg effect in cancer
Tumour cells carries out aerobic glycolysis and still produces lactate and then continues into the TCA cycle
Only 6 ATP compared to usual 31 ATP
What is oxaloacetate
4 carbon molecule
Feeds back into TCA cycle to combine with a 2 carbon molecule forming citric acid
What can be used to produce ATP
Lactate
Fatty acids
Amino acids
Are fatty acids oxidised quickly
Beta oxidation pathway is slow
