Energy Metabolism

Question 1

How do cells obtain energy?

Answer 1

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

Question 2

What is ATP composed of?

Answer 2

adenine (purine base)
ribose
three phosphate groups

Question 3

Glucose metabolism is a series of linked pathways

Answer 3

Glycolysis
Krebs’ cycle
Oxidative phosphorylation

Question 4

Glycolysis

Answer 4

anaerobic break down of glucose to pyruvate. Small amount of ATP generated by substrate level phosphorylation

Question 5

Krebs’ cycle

Answer 5

oxidation of Acetyl CoA to CO2.

generates coenzymes: NADH and FADH2

Question 6

Oxidative phosphorylation

Answer 6

transduction of energy derived from fuel oxidation to high energy phosphate.

Generates large amounts of ATP.

Question 7

Where does glycolysis occur?

Answer 7

Occurs in cytosol under anaerobic conditions
Cytosol is the intra cellular fluid

Can occur with or without oxygen

Question 8

Why does glycolysis occur?

Answer 8

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

Question 9

In glycolysis what is one molecule of glucose broken down into?

Answer 9

2 molecules of 3C pyruvate (C3H4O3)

2 NADH and 2H+

2ATP

Question 10

ATP yield in glycolysis relative to ATP yield in aerobic respiration

Answer 10

Glycolysis produces ATP much faster that aerobic respiration, but its incomplete oxidation, so the ATP yield is much less

Question 11

Red blood cells way to produce ATP

Answer 11

Only metabolic pathway within RBC

RBC lack mitochondria so glycolysis is their only way of producing energy

Question 12

How and why is glycolysis used in exercising skeletal muscle

Answer 12

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

Question 13

How is pyruvate converted into lactate?

Answer 13

Pyruvate is converted to lactate in a reaction catalysed by enzyme lactate dehydrogenase

Question 14

Glycolysis step 1 (preperative phase)

Answer 14

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

Question 15

Step 2

Answer 15

Conversion of glucose 6 phosphate to fructose 6 phosphate by phosphoglucoisomerase, reversible reaction

Question 16

Step 3

Answer 16

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

Question 17

Step 4: the splitting stage

Answer 17

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

Question 18

Step 5: the ATP generating phase

Answer 18

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+

Question 19

Step 6

Answer 19

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

Question 20

Step 7

Answer 20

Phosphoglyceromutase shifts the phosphate from the C3 to the C2 position creating 2 phosphoglycerate.

Question 21

Step 8

Answer 21

Dehydration reaction catalysed by enolase to generate high energy compound phosphoenolpyruvate

This therefore releases 2 molecules of water

Question 22

Step 9

Answer 22

Phosphoenolpyruvate is used by pyruvate kinase to phosphorylate ADP yielding the 2nd ATP molecule resulting in the product of glycolysis , the three carbon pyruvate

Question 23

Regulation of glycolysis

Answer 23

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

Question 24

Allosteric regulation

Answer 24

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

Question 25

Hormonal (insulin and glucagon) regulation

Answer 25

↑s or ↓ gene expression of the enzyme Indirect route- through affecting regulatory molecules ( usually kinases or phosphatases) ↑s or ↓ enzyme activity

Question 26

What are glycolysis enzymes sensitive to?

Answer 26

The cell’s energy levels

Question 27

What is PFK regulated by?

Answer 27

PFK is regulated by ATP, an ADP derivative called adenosine monophosphate (AMP), citrate and Fructose 2, 6 bisphosphate

Question 28

ATP as a regulator

Answer 28

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

Question 29

Adenosine monophosphate (AMP) as a regulator

Answer 29

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

Question 30

Citrate as a regulator

Answer 30

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.

Question 31

Fructose-2,6-bisphosphate as a regulator

Answer 31

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

Question 32

Fate of pyruvate under anaerobic conditions

Answer 32

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.

Question 33

Fate of pyruvate in aerobic conditions

Answer 33

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

Question 34

Irreversible reaction of pyruvate to Acetyl-CoA

Answer 34

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

Question 35

Krebs’ Cycle

Answer 35

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

Question 36

Where does the Krebs’ cycle take place?

Answer 36

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.

Question 37

Why does the Krebs’ cycle occur?

Answer 37

Generates LOTS of energy (ATP) Provides final common pathway for oxidation of carbohydrates, fat & protein via acetyl CoA Produces intermediates for other metabolic pathways

Question 38

Step 1 of Krebs’ cycle and Mnemonic “Can I Keep Selling Socks For Money Officer?”

Answer 38

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?

Question 39

Step 2

Answer 39

Citrate is converted to Isocitrate by aconitase

Question 40

Step 3

Answer 40

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

Question 41

Step 4

Answer 41

Alpha-ketoglutarate is converted into succinyl CoA by alpha-ketoglutarate dehydrogenase and addition of CoA and removal of CO2 NADH + H+ produced

Question 42

Step 5

Answer 42

Succinyl CoA is converted into Succinate as CoA is subtracted and GDP is phosphorylated by succinyl CoA synthetase. Generates ATP through substrate level phosphorylation

Question 43

Step 6

Answer 43

Succinate is oxidized to Fumarate by succinate dehydrogenase ( complex II part of the ETC), reducing FAD in the process FADH2 is produced

Question 44

Step 7

Answer 44

Fumarate is converted to Malate by fumarase, adding water in the process

Question 45

Step 8

Answer 45

Malate is converted back to oxaloacetate by malate dehydrogenase and is further oxidized, and NAD+ is reduced NADH + H+ produced

Question 46

Regulation of pyruvate dehydrogenase

Answer 46

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

Question 47

Regulation of Citrate synthase

Answer 47

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

Question 48

Regulation of Isocitrate DH

Answer 48

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

Question 49

Regulation of alpha-ketoglutarate DH

Answer 49

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

Question 50

Where does 4-Oxidative phosphorylation take place?

Answer 50

Occurs in the inner mitochondrial membranes, aerobic conditions

Question 51

Why does 4-Oxidative phosphorylation occur?

Answer 51

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

Question 52

Aim of Electron Transport system

Answer 52

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)

Question 53

What is the electron transport chain?

Answer 53

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.

Question 54

What type of reactions take place in the ETC?

Answer 54

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.

Question 55

What happens at the electron transport chain?

Answer 55

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.

Question 56

What is oxygens role in the process?

Answer 56

Oxygen sits at the end of the electron transport chain, where it accepts electrons and then combines with hydrogen ions (protons) to form water

Question 57

ENZYMES: Kinase

Answer 57

enzyme that adds/removes phosphate group to things from an ATP

Question 58

Isomerase

Answer 58

enzyme that rearranges structure of substrate without changing the molecular formula. (Similar to a mutase)

Question 59

Aldolase

Answer 59

enzyme that creates or breaks carbon-carbon bonds

Question 60

Dehydrogenase

Answer 60

enzyme that moves hydride ion (H-) to an electron acceptor e.g. (NAD+ of FAD+)

Question 61

Enolase

Answer 61

enzyme that produces a carbon=carbon double bond by removing a hydroxyl group (OH)