What pathway do we get our energy from? Anabolic or catabolic?
– catabolic pathway
What are the three general metabolic pathways?
– catabolic, anabolic and generation of metabolites that control cell activity
Fatty acid molecules are broken down by what process?
– beta oxidation
What is the name of the reduced metabolites which carry electrons to the TCA?
NADH and FADH2
What serves as the ultimate electron acceptor?
Ultimately, what happens to oxygen?
– it binds with H and forms H2O
Why does oxygen act like a scavenger/ waste disposal?
– it accepts things the cell is trying to get rid of
Name two examples of where metabolism occurs in the absence of oxygen
– exercise, within the kidney
What is the proper name for NAD?
– Nicotinamide adenine dinucleotide
What is the proper name for FADH?
– Flavine adenine dinucleotide
What is the overall cellular energy state defined by?
– the ratio of ATP to ADP
Nutrient metabolism generates reducing equivalents in the form of:
– NADH and FADH2
How many carbons are there in acetyl CoA?
In what form are the 2 carbons of acetyl coA released from the cell?
In order for the TCA to start, what resulting molecules of degradative reactions must be present?
– acetly coA (from pyruvate) and reducing equivalents (NADH and FADH2)
In the Krebs cycle, what is acetate metabolized to?
In the Krebs cycle, acetate is metabolized to CO2 and what is generated?
– additional reducing equivalents
What is generated from the shuttling of reducing equivalents along the electron transport chain?
What is the goal of the ETC?
– To generate electrons which then can be used to generate ATP which can be used in the cell
What nutrients can form acetyl coA?
– fatty acids, amino acids and glucose
What happens to the byproducts of the beta oxidation of fatty acids within the mitochondrion?
– they are sent out of the mitochondrion to be used in the synthesis of lipds (e.g., cholesterol)
What is the reverse pathway of glycolysis?
What are the two uses for amp?
– energy source and for cellular signaling
What other compounds contain high energy phosphate bonds?
– GTP (can be bound to cytoskeletal elements), UTP and CTP (play a role in DNA synthesis)
What is the sugar present in ATP?
What is the name for the high energy linkages between the phosphates in ATP?
– anhydride linkages
What is the linkage between the ribose sugar and phosphate in ATP?
– ester linkage
How can we define Gibbs’ free energy?
– maximum amount of energy that can be obtained from a reaction at a constant temperature and pressure (how much it costs to carry a particular reaction)
If a reaction has a negative free energy, what can we know about the reaction?
– It occurs spontaneously
If a reaction has a negative free energy, what can we know about the reaction?
– It occurs spontaneously
How is the reaction of glucose-6-phosphate (Glc-6-P) written?
– Glc – 6 + H2O to Glucose + Pi (this rxn can occur without added energy, even though this rxn releases energy, we say it is nagative)
If delta G is positive then:
– the rxn cannot occur spontaneously
Free energy could simply be defines as:
– how much it costs to carry out a particular type of rxn
Free energy changes are:
– additive (this is an exception to the basic free energy rule)
The direction of a rxn pathway depends on:
– the sum of the free energy changes
If cells contain more NAD+ than NAD:
– the reaction can occur
If GTP is 100 fold higher than GMP:
– the reaction can occur
Will this reaction work?: Delta G = +5.5 kcal/mol
– NOT FAVORABLE
Will this reaction work?: Delta G = -12.9 kcal/mol
– VERY FAVORABLE
Will this reaction work?: Delta G = +0.7 kcal/mol
– NOT FAVORABLE
– When you add all the above rxns together, you get about – 6.7, so the rxn should occur
Ethanlol can be an energy source that can make acetly coA
When you drink it, it is broken down into a number of metabolites
Ethanol to acetylyaldehyde to acetic acid to acetyly coA (NAD – NADH / GTP – GMP drives the rxn forward)
What is the main reason ATP helps rxns so well?
– b/c the conversion of ATP-ADP has a very high negative free energy
In an otherwise unfavorable reaction, what can drive them forward?
– The free energy of high-energy bonds
Nearly all biosynthetic pathways are thermodynamically unfavorable. (would not occur spontaneously)
– ATP provides this help
What does it mean by coupling reactions?
– You can make an unfavorable rxn favorable by pairing a favorable and unfavorable rxn to
drive it forward (with hydrolysis of high-energy bonds)
How is ATP used for metabolic work?
– High-energy linkages are broken and ATP is converted to ADP or to AMP.
Is ATP necessarily part of the reaction?
– No. It provides the energy so the rxn can be carried out
Give one example of coupling phosphoylation with ATP hydrolysis
1. Glc + Pi to Glc -6-P + H2O = +3.3 kcal/mol
2. ATP + H2O to ADP + Pi = -7.3 kcal/mol
Net: Glc + ATP to Glc -6-P + ADP = -4.0 kcal/mol
How does ATP hydrolysis work to drive reaxtions forward?
– coupled phosphorylations
The majority of energy production comes via the oxidation of fuel.
What four main fuel sources are there?
– Amino acids, Fatty acids, Lactate, Glucose
Describe the two stages of fuel oxidation
STAGE 1. Production of reduced nucleotide coenzymes (NADH, FADH2) (CO2 produced)
STAGE 2. Use of free energy from oxidation to produce ATP (O2 added, H2O produced)
Result = ATP (high-energy phosphate)
What are the three main metabolic sources of Acetyl-CoA?
– Carbohydrates, Lipids, and Proteins
(acetyl-coA is the common product of these catabolic pathways)
(carbohyrates to puruvic acid, Lipids to fatty acids, Proteins to amino acids)
What does acetly-coA transport into the TCA?
– carbon atoms (forms a thioester bond between CoA and acetic acid)
Thioester Bond Energy
Coenzyme A forms high-energy thioester bonds with acyl groups. The energy-rich nature of thioesters, as compared
with ordinary esters, is related primarily to resonance stabilization (Figure 14.9). Most esters have
two resonance forms (Figure 14.9). Stabilization involves -electron overlap, giving partial double-bond character to the C-O link.
In thioesters, the larger atomic size of S (as compared with O) reduces the -electron overlap between C and S, so that the C=S structure does not contribute significantly to resonance stabilization. As a result, the thioester is destabilized relative to an ester, thus releasing more energy on hydrolysis.The lack of double-bond character in the C-S bond of acyl-CoAs makes this bond weaker than the corresponding C-O bond
in ordinary esters, thus making the thioalkoxide ion, a good leaving group in nucleophilic displacement reactions. Consequently, the acyl group is readily transferred to other metabolites, as occurs in the first reaction of the citric acid cycle (Figure 14.3).
What is the reason that we have alternative pathways for producing acetyl-coA?
– we need to continuously make it
Overview of the breakdown of pyruvic acid to acetyly-coA
– Pyruvic acid + Coenzyme A to Acetyl-coA + 2CO2 + 4H
(remember that other compounds can be broken down into acetyl-coA, but this pathway is important for glycolysis)
Fermentation vs glycolysis?
– Fermentation is anaerobic respiration. Glycolysis is part of aerobic respiration. The pathways for
both processes are identical up to the pyruvate step. In aerobic respiration, pyruvate enters
Krebs Cycle, but in fermentation, pyruvate has to be converted to ethanol or lactic acid to regenerate NAD+.
Is glycolysis anaerobic?
– Yes! Glycolysis is an anaerobic metabolism as it doesn’t require oxygen. But it occurs for both aerobic and anaerobic
respiration. It is only the process after glycolysis that differenciates the two from each other. In respiration, pyruvate
would be completely broken down to water and carbon dioxide, but in anaerobic respiration it would be reduced to lactic acid
or broken down to ethanol and carbon dioxide, depending on whether it is alcoholic or lactic acid fermentaion.
Major nutrients are degraded to
(but some are reversible pathways)
Name four nutrients that are converted to acetyl-coA
– Carbohydrates, fats, proteins, and alcohol
What happens to the carbons of acetyl-coA in the mitochondria?
What is then transferred to coenzymes?
– they are OXIDIZED to CO2, via the TCA
(electrons are transferred to the respiratory chain)
Under low energy conditions, how is fat utilized in the brain?
– via ketone bodies (they can generate acetyl-coA)
Acetyl-coA can also act reversely to make ketone bodies, fatty acids (leading to trigycerides), and amino acids
How much of the cell’s energy comes from the mitochondria?
– approximately 95%
How many ATP does glycolysis yield?
– 2 ATP (anaerobic)
For every mole of glucose, how much ATP is generated from the mitochondria?
– 36 ATP (aerobic)
What happens after pyruvate and fatty acids and amino acids are transported to the matrix?
– They are oxidized to generate CO2, and the reduced nucleotide coenzymes NADH and FADH2
What is unusual about the mitochondria? (compared to the ER or Golgi)
– has seperate DNA, makes own ribosomes, double membrane
What is the only other organ in the body with a double membrane?
Where is pyruvic acid converted to acetyl-coA?
– in the matrix of the mitochondria
Fatty acids and amino acids are converted to what in the mitochondria?
– acetoacetic acid
How many molecules of CO2 are produced for each acetyl residue?
What is the final common pathway for the oxidation of all major nutrients?
Where does the final oxidation take place?
– mitochondrial matrix
What does the first reaction of the TCA form?
– citrate (6-carbons)
Describe the formation of citrate in the TCA
– the acetyl group of acetyl-coA reacts with the four-carbon compound oxaloacetate to
form the six-carbon compound citrate
acetyl group (reacts with) 4C oxaloacetate (to form) 6C citrate
What enzyme catalyzes the irreversible (cannot go in the opposite direction) reaction of oxaloacetate to citrate?
– citrate synthase
What is done then with citrate?
– the remaining reactions regenerate oxaloacetate from citrate, with 2 carbons released as CO2
– The term amphibolic is used to describe a biochemical pathway that involves both catabolism and anabolism –
Why is the TCA described as being amphibolic?
– b/c of its catabolic and anabolic nature – energy production and bisynthesis
(not just making electrons, but also synthesizing other products)
NADH produces 3 ATP during the ETC (Electron Transport Chain) with oxidative phosphorylation because NADH gives up its electron to Complex I,
which is at a higher energy level than the other Complexes. When Complex I transfers the electron to Complex III, energy is given off to
pump protons across the membrane, creating a gradient. The electron moves again to Complex IV and again pumps more electrons across the
membrane. Because NADH started with Complex I, it had more chances to pump more protons across the gradient, which powers the ATP synthase
and gives us 3 ATP per molecule of NADH. FADH2 produces 2 ATP during the ETC because it gives up its electron to Complex II, bypassing Complex I.
By bypassing Complex I, we missed a chance to pump protons across the membrane,
so less protons have been pumped by the time we get to Complex IV. Protons still have been pumped,
enough to fuel 2 ATP created by ATP synthase.
What is the intermediate for carbohydrates in the TCA?
What is the intermediate for proteins in the TCA?
– aspartate or glutamate
What is the intermediate for fatty acids in the TCA?
Why are redox (ELECTRON GAIN) reactions involving coenzymes important in cellular respiration?
– FADH2 and NADH donate H+ & electrons to the electron transport chain (ETC)
in the inner membrane of the mitochondria. As the electrons pass from one carrier to
another (Redox – reduction = electron gain and Oxidation = electron loss) H+ are pumped into
the space between the inner and outer membranes creating a H+ gradient with the stroma of the
mitochondria. The H+ pass through the membrane bound ATP synthase to the matrix and ATP is produced.
So, if there were no redox reactions in the ETS there would be no large scale generation of ATP –
the whole point of respiration.
The byproducts of most catabolic processes are NADH and [FADH2] which are the reduced forms.
Metabolic processes use NADH and [FADH2] to transport electrons in the form of hydride ions (H-).
LETS TALK ABOUT REDOX COENZYMES
What are coenzymes?
– NAFH and FADH2 can also act as coenzymes, helping reactions to occur.
They are important for synthesizing molecules.
Regarding the structure of redox coenzymes, describe how NAD+ accepts protons and electrons
– it accepts one proton (H), and two electrons
Regarding the structure of redox coenzymes, describe how FAD accepts protons and electrons
– it accepts two proton (H2), and two electrons
What is the proper name for FMN?
– flavine mononucleotide
What is NADPH?
– It does not carry electrons like NADH. It is a coenzyme.It works in reductive biosynthesis.
– Both NADPH and NADH and FADH2 carry reducing equivalents, but they are used differently.
What are the major electron acceptors in fuel catabolism?
– NAD+ and FAD
What is the primary electron donor in reductive biosynthesis?
– NADPH (maintains glutathione function – the superhero of antioxidants!!)
What is NADPH, what is the difference between NADP+ and NADPH and how is NADP+ turned into NADPH? ?
– NADPH is a reducing agent. NADP+ is the oxidized version of NADPH. When an
H with 2 electrons are added to NADP+ you get NADPH which is said to be reduced.
NADPH is a reducing agent that is used in anabolic reactions (REDUCTIVE BIOSYNTHESIS). NADPH stands for Nicotinamide
Adenine Dinucleotide Phosphate.
What are redox reactions?
– combination of reduction and oxidation
How do we eventually generate 12 ATP from the TCA?
– 3 NADH (3 ATP each), 1 FADH (2 ATP), 1 GTP (1 ATP)
Why is it that NADH and FADH2 look different, but both carry 2 electrons?
– NADH carries 2 electrons on its H, but FADH2, takes 2 H, and carries one electron on each H
Oxidative phosphorylation allows for the conversion of what?
– FAD, NAD+
Do NADH and FADH2 carry the same amount of reducing potential?
– YES!!! (2 electrons, but they generate different amounts of ATP)
How many ATP are generated IN the TCA cycle?
One turn of the TCA cycle generates how many ATP?
1. NADH – 3 ATP x 3 = 9 ATP
2. FADH2 – 2 ATP x 1 = 2 ATP
3. GTP – 1 ATP x 1 = 1 ATP
12 ATP (3 cycles so about 36 ATP, plus 2 from glycolysis = 38 ATP)
What are the two tricarboxylic acids in the TCA?
– Citrate and Isocitrate
What rxns make NADH in TCA?
1. Isocitrate to a-Keto glutarate (+ CO2)
2. a-keto glutarate to succinyl-coA (+CO2)
3. Malate to Oxaloacetate
What rxns make FADH in the TCA?
1. succinate to Fumarate
What rxn makes GTP in the TCA?
1. Succinyl-coA to Succinate
1. Oxaloacetate (4C) to Citrate (6C)
– Acetyl-coA (2C) AND Citrate synthase
2. Citrate to Isocitrate
3. Isocitrate to a-ketoglutarate
– isocitrate dehydrogenase (+ NADH and CO2)
4. a-ketoglutarate to succinyl-coA
– a-ketoglutarate dehydrogenase (+NADH and CO2)
5. succinyl-coA to Succinate
– succinyl-coA synthase (+ GTP)
6. Succinate to Fumarate
– succinate dehydrogenase (+ Qh2)
7. Fumarate to Malate
– Fumarase (+ water)
8. Malate to Oxaloacetate
– Malate dehydrogenase (+ NADH)
Cellular respiration formula
C6 H12 O6 + 6 02 ———-> 6 H2O+ 6 CO2
Each molecule that enters the TCA produces the equivalent of how many ATP?
Substrate-level phosphorylation produces GTP…and…
– It is readily converted to ATP
What is the total energy produced by oxidation of one mole of glucose
through the CAC (citric acid cycle)
– 36-38 moles of ATP (3 cycles of CAC)
Gylcolysis produces how many ATP, NADH?
– Glycolysis produces 4 ATP’s and 2 NADH, but uses 2 ATP’s
in the process for a net of 2 ATP and 2 NADH
NOTE: this process does not require O2 and does not yield much energy
Net Engergy Production from Aerobic Respiration
Glycolysis: 2 ATP
Krebs Cycle: 2 ATP
Electron Transport Phosphorylation: 32 ATP
Each NADH produced in Glycolysis is worth 2 ATP (2 x 2 = 4) – the NADH is worth 3 ATP,
but it costs an ATP to transport the NADH into the mitochondria, so there is a net gain of
2 ATP for each NADH produced in gylcolysis
Each NADH produced in the conversion of pyruvate to acetyl
COA and Krebs Cycle is worth 3 ATP (8 x 3 = 24)
Each FADH2 is worth 2 ATP (2 x 2 = 4)
4 + 24 + 4 = 32
Net Energy Production: 36 ATP!
High levels of what intermediate metabolites stimulate ATP production?
– NAD+, AMP, coA
If there are low levels of THIS, ATP cannot be made
There is normally more NAD+ inside the cell than NADH, so when there is not enough
NADH, more NAD+ is made
Remember – the accumulation of intermediate metabolites is NOT desirable
Increased ATP untilization causes increased ATP production
Which stage is called THE PACEMAKER stage?
– isocitrate to a-ketoglutarate
– we don’t want to make excess levels of ATP becuase it is a waste of energy ‘expensive’
The formation of Citrate can be inhibited when?
– when there are high levels of ATP
Remember: Intermediates can used by other pathways, but their levels must be maintained for the TCA cycle to continue
– The TCA also interacts with other pathways
Which major section sof TCA are involved in other pathways?
1. Oxaloacetate (used to make Glucose, and aspartate can make oxaloacetate and visa versa)
2. Citrate (used to make acetyl-coA, which makes fatty acids)
3. a-ketoglutarate (used to make glutamate, which can make glutamine)
4. succinyl-coA (used to make heme)
Remember: the TCA is NOT a closed system
What are the most important three enzymes in the TCA?
– citrate synthase (oxaloacetate to citrate)
– isocitrate dehydrogenase (isocitrate to a-ketoglutarate) – IDH is the PACEMAKER STAGE
– a-ketoglutarate dehydrogenase (a-ketoglutarate to succinyl-coA)