Energy Metabolism Flashcards
How do cells obtain energy?
Cells can get energy from nutrients or fuels such as carbohydrates, Proteins and lipids
Cells conserve some of the energy to generate a metabolic currency Adenosine triphosphate (ATP)
ATP is the currency of metabolic energy
What is ATP composed of?
adenine (purine base)
ribose
three phosphate groups
Glucose metabolism is a series of linked pathways
Glycolysis
Krebs’ cycle
Oxidative phosphorylation
Glycolysis
anaerobic break down of glucose to pyruvate. Small amount of ATP generated by substrate level phosphorylation
Krebs’ cycle
oxidation of Acetyl CoA to CO2.
generates coenzymes: NADH and FADH2
Oxidative phosphorylation
transduction of energy derived from fuel oxidation to high energy phosphate.
Generates large amounts of ATP.
Where does glycolysis occur?
Occurs in cytosol under anaerobic conditions
Cytosol is the intra cellular fluid
Can occur with or without oxygen
Why does glycolysis occur?
Emergency energy producing pathway when oxygen is limiting
• RBCs and exercising skeletal muscle
Generates precursors for biosynthesis
• G-6-P (glucose 6 phosphate) converted to
• ribose-5-P (nucleotides) via pentose phosphate pathway
• G-1-P for glycogen synthesis
Pyruvate
• transaminated to alanine- the transfer of an amino group from another molecule to pyruvate to produce alanine
• substrate for fatty acid synthesis
Glycerol-3-P is backbone of triglycerides
In glycolysis what is one molecule of glucose broken down into?
2 molecules of 3C pyruvate (C3H4O3)
2 NADH and 2H+
2ATP
ATP yield in glycolysis relative to ATP yield in aerobic respiration
Glycolysis produces ATP much faster that aerobic respiration, but its incomplete oxidation, so the ATP yield is much less
Red blood cells way to produce ATP
Only metabolic pathway within RBC
RBC lack mitochondria so glycolysis is their only way of producing energy
How and why is glycolysis used in exercising skeletal muscle
Used by Exercising skeletal muscle when oxidative metabolism can’t keep up with increased energy demand
Leads to a build up of acid
Normally the liver handles this in the cori cycle, where lactate from the blood is converted back to pyruvate and used to make new glucose molecules
The majority of cells do not have sufficient amounts of glycolytic enzymes or enough glucose to provide for glycolysis to meet the energy requirement of the cell using glycolysis alone
How is pyruvate converted into lactate?
Pyruvate is converted to lactate in a reaction catalysed by enzyme lactate dehydrogenase
Glycolysis step 1 (preperative phase)
Glucose enters cell via GLUT-1 transporters and phosphorylated to Glucose 6 phosphate, a reaction catalysed by hexokinase.
This is a one-way process that essentially traps glucose inside the cell and commits it to glycolysis.
This step is energetically unfavourable and so its is couples with ATP hydrolysis in order to use a phosphate from ATP, producing ADP and glucose-6-phosphate
Step 2
Conversion of glucose 6 phosphate to fructose 6 phosphate by phosphoglucoisomerase, reversible reaction
Step 3
A second molecule of ATP is invested to produce fructose- 1,6-bisphosphate catalysed by phosphofructokinase 1( PFK1).
This is an irreversible reaction and the stop commits glucose to glycolysis
Step 4: the splitting stage
Fructose 1,6, bisphosphate is cleaved into 3 phosphorylated 3 carbon compounds : dihydroxyacetone phosphate and glyceraldehyde-3-phosphate catalysed by fructose bisphosphate aldolase
Only glyceraldehydes continues on through the glycolytic pathway but triosphosphate isomerase catalyses the interconversion of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate
Step 5: the ATP generating phase
The aldehyde group of G3P is oxidised to a carboxyl group catalysed by glyceraldehyde 3 phosphate dehydrogenase (triose phosphate dehydrogenase).
A phosphate is also added to the glyceraldehyde-3-phosphate to produce 1,3-Bisphosphoglycerate
The coenzyme NAD is reduced to NADH producing NADH and H+
Step 6
Phosphoglycerokinase catalyses the transfer of a phosphate group from the high energy acyl phosphate of 1,3 BPG to ADP forming ATP.
The other phosphate group in 3 PG does not have enough energy to phosphorylate another ADP, so a series or isomerisation and dehydration reactions are required to convert this compound to a high energy enol phosphate
Step 7
Phosphoglyceromutase shifts the phosphate from the C3 to the C2 position creating 2 phosphoglycerate.
Step 8
Dehydration reaction catalysed by enolase to generate high energy compound phosphoenolpyruvate
This therefore releases 2 molecules of water
Step 9
Phosphoenolpyruvate is used by pyruvate kinase to phosphorylate ADP yielding the 2nd ATP molecule resulting in the product of glycolysis , the three carbon pyruvate
Regulation of glycolysis
Glycolysis is regulated allosterically at three kinase reactions.
PFK1 is the primary regulatory site of glycolysis
Phosphofructokinase-1 (PFK1) is pH dependant and is inhibited by acidic conditions- explaining why glycolysis is inhibited in acidosis
Allosteric regulation
Whatever molecule, example ATP, is going to bind to enzyme to regulate it will Bind to a non-catalytic site
Conformational change
↑s or ↓ enzymes affinity for the substrate
Hexose kinase, phosphofrictise kinase and pyruvate kinase are under allosteric control
Hormonal (insulin and glucagon) regulation
↑s or ↓ gene expression of the enzyme
Indirect route- through affecting regulatory molecules ( usually kinases or phosphatases)
↑s or ↓ enzyme activity
What are glycolysis enzymes sensitive to?
The cell’s energy levels
What is PFK regulated by?
PFK is regulated by ATP, an ADP derivative called adenosine monophosphate (AMP), citrate and Fructose 2, 6 bisphosphate
ATP as a regulator
Adenosine triphosphate (ATP) is an allosteric inhibitor (modifies the active site of
the enzyme so that the affinity for the substrate decreases) for PFK-1
Thus at low ATP levels = fast reaction speed of PFK-1 so more fructose 1,6 bisphosphate
At high ATP levels = slow reaction speed of PFK-1 so less fructose 1,6 bisphosphate
AMP opposes the allosteric inhibition by ATP
Adenosine monophosphate (AMP) as a regulator
AMP is an allosteric activator of PFK-1.
(modifies the active site of the enzyme so that the affinity for the substrate increases) of phosphofructokinase-1 (PFK-1).
AMP binds to PFK-1 resulting in a conformational change - increasing affinity of PFK-1 for fructose-6-phosphate
When ATP is used up, ADP accumulates and is converted to AMP by Adenylate kinase reaction to generate ATP.
2ADP = ATP + AMP
Increasing levels of AMP relieves the inhibition of PFK-1 by ATP
Citrate as a regulator
Citrate is the first product of the kreb’s cycle also acts allosterically inhibit PFK-1.
Increase citrate levels is a signal that the cycle does not need more fuel.
Fructose-2,6-bisphosphate as a regulator
Fructose-2,6-bisphosphate generated from Fructose-6-phosphate is the most important allosteric activator of PFK1.
Mediates effect of insulin and glucagon. It is an
Fate of pyruvate under anaerobic conditions
Lactate formation catalysed by lactate dehydrogenase
Regeneration on NAD+
Under anaerobic conditions it is deduced to lactate during anaerobic metabolism
Anaerobic conditions: the middle carbonyl in pyruvate is reduced (hydrogen added) to an alcohol group, and lactate is formed.
• The hydrogen (and energy) required for this reaction is supplied by NADH and H+, producing NAD+.
• The NAD+ produced funnels back into glycolysis to oxidize more glyceraldehyde-3-phosphate (step 6), providing a small amount of ATP. This reaction occurs in the cytosol.
If NAD+ is in short supply, glycolysis cannot continue. An alternative way to reoxidize NADH is essential because glycolysis, the only available source of fresh ATP, must continue. The reduction of pyruvate to lactate solves the problem.
Some of the lactate that is formed is released into the blood and taken up by the heart & brain where it is converted back to pyruvate and used as an energy source
Another portion of lactate is taken up by the liver where it is used as a precursor for the formation of glucose, which is then released into the blood where it becomes available as an energy source for cells.
Fate of pyruvate in aerobic conditions
Pyruvate enters the mitochondria converted to Acetyl CoA and CO2 by Pyruvate Dehydrogenase.
Acetyl CoA can enter TCA cycle for more energy production
If oxygen is available, the pyruvate can be broken down (oxidized) all the way to carbon dioxide in cellular respiration, making many molecules of ATP
Irreversible reaction of pyruvate to Acetyl-CoA
Irreversible
Catalysed by Pyruvate dehydrogenase, a multi-enzyme complex within mitochondrial matrix
Inhibited by high concentrations acetyl-CoA and NADH
Inactivated by phosphorylation
Activated by phosphate removal
Krebs’ Cycle
Also known as the citric acid cycle and the tricarboxylic acid (TCA) cycle
The primary molecule molecule entering the Kreb’s cycle is acetyl coenzyme A (acetyl CoA). It is derived from the B vitamin pantothenic acid and functions primarily to transfer acetyl groups (2 carbons), from one molecule to another.
Acetyl CoA can either be made from pyruvate (see below) or the beta-oxidation of fatty acids or from amino acid breakdown
Where does the Krebs’ cycle take place?
Occurs in mitochondrial matrix, aerobic conditions only as oxidative phosphorylation is required to covert NADH & FADH2 back to NAD+ and FAD to be used in the conversion of Isocitrate to a-Ketoglutarate and a-Ketoglutarate to Succinyl coenzyme A & Succinate to Fumarate & Malate to Oxaloacetate.
Why does the Krebs’ cycle occur?
Generates LOTS of energy (ATP)
Provides final common pathway for oxidation of carbohydrates, fat & protein via acetyl CoA
Produces intermediates for other metabolic pathways
Step 1 of Krebs’ cycle and Mnemonic “Can I Keep Selling Socks For Money Officer?”
Acetyl CoA combines with oxaloacetate to form Citrate using citrate synthase. Requires water. Releases CoA
- C - Citrate - Can
- I - Isocitrate - I
- K - a-Ketoglutarate - Keep
- S - Succinyl CoA - Selling
- S - Succinate - Socks
- F- Fumarate - For
- M - Malate - Money
- O - Oxaloacetate - Officer?
Step 2
Citrate is converted to Isocitrate by aconitase
Step 3
Isocitrate is oxidized and decarboxylated ( loose a CO2) by isocitrate dehydrogenase to form alpha-ketoglutarate
NADH + H+ produced
REMEMBER AS: first time oxidation happens in Krebs’ cycle, decarboxylation also occurs
Step 4
Alpha-ketoglutarate is converted into succinyl CoA by alpha-ketoglutarate dehydrogenase and addition of CoA and removal of CO2
NADH + H+ produced
Step 5
Succinyl CoA is converted into Succinate as CoA is subtracted and GDP is phosphorylated by succinyl CoA synthetase.
Generates ATP through substrate level phosphorylation
Step 6
Succinate is oxidized to Fumarate by succinate dehydrogenase ( complex II part of the ETC), reducing FAD in the process
FADH2 is produced
Step 7
Fumarate is converted to Malate by fumarase, adding water in the process
Step 8
Malate is converted back to oxaloacetate by malate dehydrogenase and is further oxidized, and NAD+ is reduced
NADH + H+ produced
Regulation of pyruvate dehydrogenase
Conversion of pyruvate is converted to acetyl CoA. Is an irreversible and tightly regulated to control how much fuels enteres the kreb’s cycle
ATP and NADH negatively inhibit pyruvate dehydrogenase. ADP activates it
Pyruvate dehydrogenase is also activated by its substrate, pyruvate, and inhibited by its product, acetyl CoA.
This ensures that acetyl CoA is made only when it’s needed
Regulation of Citrate synthase
ATP and NADH allosterically inhibit citrate synthase. Reduce affinity of citrate synthase for its substrates
Succinyl Co-A competitively inhibits citrate synthase
Citrate inhibits citrate synthase
ADP activator
Increase citrate inhibition citrate synthase, reduces speed of cycle
Regulation of Isocitrate DH
A key rate limiting enzyme of Krebs’ Cycle
In states of increased oxidative phosphorylation demands, the rate of the Krebs’ Cycle reactions is increased
However, limited by product inhibition of citrate synthase
ADP is an activator
ATP and NADH are inhibitors
Isocitrate dehydrogenase activation leads to a decrease in citrate
Citrate synthase reaction rate increased
Regulation of alpha-ketoglutarate DH
Inhibited by its products NADH and succinyl-CoA
Also inhibited by GTP, ATP, and reactive oxygen species (ROS)
ROS are also produced by α-ketoglutarate DH
Activated by Ca2+ may be useful in generating ATP during intense muscle exercise
Where does 4-Oxidative phosphorylation take place?
Occurs in the inner mitochondrial membranes, aerobic conditions
Why does 4-Oxidative phosphorylation occur?
Releases the majority of energy during cellular respiration
Reduced NADH or FADH2 from glycolysis and Kreb’s cycle are oxidised and their electrons passed to components of the electron transport chain (ETC). These are a series of carriers embedded in the inner mitochondrial membrane. The final electron acceptor is O2.
Energy released is trapped to generate ATP
Aim of Electron Transport system
Utilize the protons and electrons that the coenzymes (NAD+ and FAD) “picked up” during glycolysis (NAD+ only) and Kreb’s cycle (both NAD+ and FAD)
What is the electron transport chain?
ETC is a series of compounds that transfer electrons form elctron donors to electron acceptors via redox
Cytochromes (contain iron and copper co-factors, structure resembles the red iron- congaing haemoglobin) and associated proteins embedded in the inner mitochondrial membrane surface form the components of the electron transport chain.
What type of reactions take place in the ETC?
Chemiosmotic process where energy for ATP synthesis is provided by an electrochemical gradient across the inner mitochondrial membrane
In chemiosmosis, the energy stored in the gradient is used to make ATP.
What happens at the electron transport chain?
Two electrons from hydrogen atoms are initially transferred either from NADH + H+ or FADH2 to one of the protein in the electron transport chain. These electrons are then successively transferred to other compounds in the chain redox reactions, until the electrons are finally transferred to molecular oxygen, which then combines with hydrogen ions (protons) to form water.
• These hydrogen ions, like the electrons, come from free hydrogen ions and the hydrogen-bearing coenzymes (NADH and FADH2), that had been released earlier in the electron transport chain when the electrons from the hydrogen atoms were transferred to the cytochromes.
• IMPORTANTLY in addition to transferring the coenzyme hydrogens to water, this process also regenerates the hydrogen-free forms of the coenzymes (NAD+ & FAD), which can then become available to accept two more hydrogens from intermediates in the Kreb’s cycle, glycolysis or beta-oxidation.
• Thus, the electron transport chain provides the aerobic mechanism for regenerating the hydrogen-free form of the coenzymes
• At certain steps along the electron transport chain, small amounts of energy are released. As electrons are transferred from one protein to another alone the chain, some of the energy released used by the cytochromes to pump hydrogen ions from the matrix into the intermembranal space - the compartment between the inner and outer mitochondrial membranes
• This creates a source of potential energy in the form of a hydrogen-ion- concentration gradient across the membrane.
• Embedded in the inner mitochondrial membrane are enzymes called ATP synthase. This enzyme forms a channel in the membrane, allowing hydrogen ion to flow back into the matrix via chemiosmosis - moving from an area of high concentration of hydrogen ions to an area of low concentration. During this process, the energy of the concentration gradient is converted into chemical bond energy by ATP synthase, which then catalyses the formation of ATP from ADP and Pi.
• The transfer of electrons to oxygen produces on average around 2.5 and 1.5 molecules of ATP for each molecule of NADH + H+ & FADH2 respectively.
What is oxygens role in the process?
Oxygen sits at the end of the electron transport chain, where it accepts electrons and then combines with hydrogen ions (protons) to form water
ENZYMES: Kinase
enzyme that adds/removes phosphate group to things from an ATP
Isomerase
enzyme that rearranges structure of substrate without changing the molecular formula. (Similar to a mutase)
Aldolase
enzyme that creates or breaks carbon-carbon bonds
Dehydrogenase
enzyme that moves hydride ion (H-) to an electron acceptor e.g. (NAD+ of FAD+)
Enolase
enzyme that produces a carbon=carbon double bond by removing a hydroxyl group (OH)
