CITRIC ACID CYCLE

Question 1

Cyclic pathway; end and initial product is the Oxaloacetate
Important biochemical pathway in body; generates energy we need and generates CO2

Answer 1

The Citric Acid Cycle aka Kreb’s Cycle

Question 2

Totality of biochemical reactions
Biochemical reactions: takes place in a living organism
Once food goes into the body they will undergo different biochemical reactions depends on what the body needs and will be used for different processes

Answer 2

Metabolism
Body: 6 tons of solid food, 10k gallon of water (70%)
Metabolic process: biochemical reactions happens inside the cell (intracellular)

Question 3

releases energy during the process
All metabolic breaks large molecules into smaller molecules = energy is released
Ex: oxidation of glucose

Answer 3

Catabolism

Question 4

small molecules are combined to form larger molecules
Requires energy during the reaction
Ex: Synthesis of proteins from monomer units (AA)

Answer 4

Anabolism

Question 5

linear and cyclic pathway
Series of molecular or biochemical reactions organized in sequence
Converting starting material to an end product

Answer 5

Metabolic pathways

Question 6

Metabolic pathways:
there is starting and specific final product
Starting point —> final product
Series of reactions that generates a final product

Answer 6

Linear pathway

Question 7

Metabolic pathways:
first reactant during the pathway is also the final product
Series of reaction which generates the first reactant
End product is also first reactant used in the metabolic pathway

Answer 7

Cyclic pathway

Question 8

A if only the first statement is true
B if only the second statement is true
C if both of the statements are true
D if neither of the statements is true

1. Energy needed to run human body is obtained from food
2. Multi-step process involves several different catabolic pathways

Answer 8

C

Question 9

A if only the first statement is true
B if only the second statement is true
C if both of the statements are true
D if neither of the statements is true

1. Nucleus is where most of the important biochemical process takes place
2. Matrix of the mitochondria is the site of ATP production

Answer 9

B

Question 10

Four general stages in the Biochemical Energy Production Process:

Answer 10

Stage 1: Digestion
Stage 2: Acetyl group formation
Stage 3: Citric acid cycle; Kreb’s cycle
Stage 4: Electron transport chain (respiratory chain; ETC) and oxidative phosphorylation

Question 11

Begins in mouth (saliva; salivary amylase) contains starch digesting enzymes)
Continues in the stomach (gastric juice; pepsin)
Completed in small intestine (trypsin, other bile salts)
Results in small molecules that can cross intestinal membrane into the blood

Answer 11

Stage 1: Digestion
(not considered part of metabolism; extracellular)

End Products of digestion: monomer units of macromolecules
Glucose and monosaccharides from carbohydrates
Amino acids from proteins
Fatty acids and glycerol from fats and oils
(these end products are small enough to be absorbed by the body and pass across the intestinal membrane and go to the circulation
The digestion products are absorbed into the blood and transported to body’s cells)

Question 12

Biochemical reactions start inside the cells; primarily mitochondria in most cases
Biochemical processes happen inside the cell
Small molecules from Stage 1 are further oxidized

Answer 12

Stage 2: Acetyl Group Formation
(metabolic processes starts)

End product of these oxidations is acetyl CoA: 2 C acetyl units and reduced coenzyme (NADH)
Involves numerous reactions: occur both in
Cytosol: glucose metabolism
Mitochondria: fatty acid metabolism of the cells.

Question 13

First intermediate of the cycle is citric acid; Therefore designated as Citric acid cycle
First product formed during the process: citric acid = citrate
Acetyl group is oxidized to produce CO2 and energy (reduced coenzyme)
Reduced coenzyme (NADH and FADH2): have equivalent ATP molecules
The carbon dioxide (CO2) we exhale comes primarily from this stage

Answer 13

Stage 3: Citric Acid Cycle
(takes place inside the Mitochondria)

Most energy is trapped in reduced coenzymes NADH and FADH2 (flavin)
Some energy produced in this stage is lost in the form of heat
Final product: CO2 and NADH AND FADH2
There are different steps which could yield CO2 and reduced coenzymes

Question 14

Takes place in Mitochondria
NADH and FADH2 (reduced coenzymes) are oxidized to release H+ and electrons
Needed to produce ATP molecules where are primarily the energy carriers in metabolic pathways
H+ are transported to the intermembrane space in mitochondria

Answer 14

Stage 4: Electron Transport Chain and Oxidative Phosphorylation

Electrons are transferred to O2 and O2 is reduced to H2O
H+ ion re-enter the mitochondrial matrix drive ATP- synthase reaction to produce ATP
Final product: ATP and H2O
ATP is the primary energy carrier in metabolic pathways

Question 15

tricarboxylic acid cycle (TCA) or Krebs cycle
Named after Hans Adolf Krebs who elucidated this pathway: Nobel price guarantee in medicine
Series of biochemical reactions in which the acetyl portion of acetyl CoA is oxidized to carbon dioxide and the reduced coenzymes; FADH2 and NADH, are produced
Utilizes the final product of Stage 2 as the initial reactant with Oxaloacetate

Answer 15

Citric Acid Cycle

The citric acid cycle also produces 2 ATP by substrate level phosphorylation from GTP (guanosine triphosphate; similar to ATP)
Mainly a cycle with 8 steps: takes place in mitochondrial matrix is because enzymes needed is found there except enzymes used in Step 6

Question 16

A if only the first statement is true
B if only the second statement is true
C if both of the statements are true
D if neither of the statements is true
Two important types of reactions in Citric Acid Cycle:

1. Oxidation of NAD+ and FAD to produce NADH and FADH2 (reduced coenzymes)
2. Hydrolysis of citric acid to produce carbon dioxide

Answer 16

A

Question 17

6 types of reactions involved in Citric Acid Cycle:

Answer 17

condensation, isomerization, oxidation, decarboxylation, phosphorylation, hydration

Question 18

4 steps that include oxidation in Citric acid cycle:

Answer 18

Steps 3, 4, 6, 8

Forms NADH and FADH2: but are not the same are formed during the reactions and also due to the reactants

Question 19

2 steps that involve decarboxylation:

Answer 19

Steps 3 and 4; forms CO2

Question 20

Final product and first reactant in CAC:

Answer 20

Oxaloacetate (together with Acetyl CoA)

Oxaloacetate is not involved in the reaction because it is the final product and first reactant of the cycle; it is not stated in the chemical reaction

Question 21

Intermediate products in CAC:

Answer 21

citrate, isocitrate, a-ketoglutarate, succinylcholine CoA, succinate, malate
Not part of the reaction because they are also consumed during the reaction

Question 22

Steps/ Reactions of the Citric Acid Cycle:
Reaction: condensation (water is involved in the reaction)
In cellular pH citric acid is present as citrate ion
Starts with using Oxaloacetate and Acetyl CoA
Atoms of Acetyl CoA is transferred to Oxaloacetate to form citrate
Enzyme: citrate synthase
Synthase: can form bond without ATP molecule

Answer 22

Step 1: Formation of Citrate; Citric Acid

Intermediate/ middle product: citryl CoA
It will then proceed to hydrolysis and is hydrolyzed it will then now form citrate
Other final product: CoA-SH + H+
Citrate: 3’ROH; don’t undergo oxidation, not readily oxidized

Question 23

Steps/ Reactions of the Citric Acid Cycle:
Reaction: isomerization but there are other reaction involved (dehydration and hydration), necessary for isomerization
Isocitrate: product from the step 1
Citrate —> isocitrate (2’ROH, less symmetrical isomer)
Enzyme: Aconitase: reacts to citrate forming an intermediate product
Citrate will be first dehydrated forming cis-aconitate

Answer 23

Step 2: Formation of Isocitrate

Intermediate product: cis-aconitate (dehydrated citrate)
Cis-aconitate will react to aconitase to form isocitrate
Aconitase will hydrate cis-aconitate; water molecules is added to cis-aconitate and forms hydroxyl and hydrogen side chain in isocitrate
Composition of side chains happened
Isomerization is done so that citrate can undergo oxidation

Question 24

Steps/ Reactions of the Citric Acid Cycle:
Reaction: oxidation
Involves oxidation-reduction (redox) as well as decarboxylation = CO2
Oxidation of isocitrate is needed to form the intermediate product
Oxalosuccinate will still undergo another reaction: decarboxylation
H+ is utilized during the reaction to form alpha-
ketoglutarate
Oxidation reactant: NAD and H+ aside from isocitrate

Answer 24

Step 3: Oxidation of Isocitrate and Formation of CO2

Intermediate product: oxalosuccinate (isocitrate + NAD and H+ with the enzyme)
Enzyme: isocitrate dehydrogenase
Name: reactant + dehydrogenase
Decarboxylation: requires H ions and CO2 will be released from the carboxyl group (COO) of the Oxalosuccinate
Once carbon dioxide is removed it will now form the final products
Final products: Alpha-Ketoglutarate, CO2, NADH (reduce form of NAD)

Question 25

Steps/ Reactions of the Citric Acid Cycle: Same reaction of Step 3: redox reaction Final products: CO2, NADH, H+, Succinyl CoA Enzyme: Alpha-Ketoglutarate dehydrogenase complex

Answer 25

Step 4: Oxidation of Alpha-Ketoglutarate and Formation of CO2 Reactant: NAD, H+, CoA-SH (formed on step 1) COO of Alpha-Ketoglutarate —> S-CoA Once Alpha-Ketoglutarate goes under oxidation it forms Succinyl CoA

Question 26

Steps/ Reactions of the Citric Acid Cycle: Succinyl CoA reactants: GDP and Pi Final product: GTP (GDP + Pi), CoA-SH GTP: similar to ATP; used to store energy in the form of high energy phosphate bond

Answer 26

Step 5: Thioester bond cleavage in Succinyl CoA and Phosphorylation of GDP Succinyl CoA Enzyme: Succinyl CoA synthetase: removes CoA by thioester bond cleavage Once cleaves it forms succinate

Question 27

Steps/ Reactions of the Citric Acid Cycle: 3rd redox reaction in the cycle Happen in the inner mitochondrial complex (succinate—> fumarate) In succinate, only a single bond is seen between 2 C (C2 and C3) COO—CH2—CH2—COO —> COO—CH=CH—COO After oxidation of succinate; double bond is formed in C2 and C3 forming fumarate Enzyme: succinate dehydrogenase: can only produce trans-isomer and not cis-double bonds

Answer 27

Step 6: Oxidation of Succinate (alkane; 2C) Reactant: FAD (flavin adenine dinucleotide), succinate FAD: oxidizing agent, forms FADH2 Only step that utilizes FAD as oxidizing agent Product: Fumarate (trans-double bond;C2 and C3), FADH2 Trans-double bond: side chains are positioned in opposite direction

Question 28

Steps/ Reactions of the Citric Acid Cycle: Reaction: Hydration Fumarate has double bond on C2 and C3 after oxidation Fumarate undergoes hydration the double bond in C2 and C3 is broken and goes back to single bond forming malate The side chain that could be bonded on the 2 C are 1H and 1OH

Answer 28

Step 7: Hydration of Fumarate Forms 2’ROH: malate (hydrated fumarate) possible to undergo oxidation Enzyme: fumarase; stereo specific, only forms L-isomer Reactant: H2O, fumarate (on the double bond of fumarate) Product: L-Malate

Question 29

Steps/ Reactions of the Citric Acid Cycle: Also the reactant used in the initial cycle Malate (oxidized) = ketone in C2 of Oxaloacetate Enzyme: malate dehydrogenase Reactant: malate

Answer 29

Step 8: Oxidation of L-Malate to Regenerate Oxaloacetate Oxidizing agent: NAD Final product: NADH, H+ (because malate is oxidized) When body needs energy or CO2 Oxaloacetate will undergo citric acid cycle together with acetyl CoA

Question 30

``` A if only the first statement is true B if only the second statement is true C if both of the statements are true D if neither of the statements is true Regulation of the Citric Acid Cycle ``` 1. The rate at which the citric acid cycle operates is not controlled by ATP and NADH levels 2. When ATP supply is high, ATP inhibits citrate synthase (Step 1 of Citric acid cycle)

Answer 30

B

Question 31

``` A if only the first statement is true B if only the second statement is true C if both of the statements are true D if neither of the statements is true Regulation of the Citric Acid Cycle ``` 1. When ATP levels are low ADP is high, ADP activates citrate synthase 2. Low ATP and ADP high = activate the cycle (citrate synthase)

Answer 31

C

Question 32

``` A if only the first statement is true B if only the second statement is true C if both of the statements are true D if neither of the statements is true Regulation of the Citric Acid Cycle ``` 1. Similarly ADP and NADH control isocitrate dehydrogenase (Step 3): 2. NADH acts as an activator and ADP as an inhibitor

Answer 32

A

Question 33

``` A if only the first statement is true B if only the second statement is true C if both of the statements are true D if neither of the statements is true Regulation of the Citric Acid Cycle ``` 1. The electron transport chain (ETC; step 4) does not facilitate the passage of electrons trapped in FADH2 and NADH during citric cycle 2. ETC is a series of biochemical reactions in which intermediate carriers (protein and non-protein) aid the transfer of electrons and hydrogen ions from NADH and FADH2

Answer 33

B

Question 34

``` A if only the first statement is true B if only the second statement is true C if both of the statements are true D if neither of the statements is true Regulation of the Citric Acid Cycle ``` 1. NADH and FADH2 are being reduced in the process it goes back to its oxidizing agent form; NAD and FAD, respectively 2. H ions utilized during reduction will be free H ions and extra electrons that could be used during the process

Answer 34

D

Question 35

``` A if only the first statement is true B if only the second statement is true C if both of the statements are true D if neither of the statements is true Regulation of the Citric Acid Cycle ``` 1. The ultimately receiver of electrons is molecular hydrogen 2. Electron transport (respiratory chain) gets its name from the fact electrons are transported to oxygen absorbed via respiration

Answer 35

B

Question 36

``` A if only the first statement is true B if only the second statement is true C if both of the statements are true D if neither of the statements is true Regulation of the Citric Acid Cycle ``` 1. Energy is used to synthesize ATP in oxidative phosphorylation 2. Reaction releases energy (exothermic reaction)

Answer 36

C

Question 37

``` A if only the first statement is true B if only the second statement is true C if both of the statements are true D if neither of the statements is true Regulation of the Citric Acid Cycle ``` 1. Energy released is coupled with the formation of three ATP molecules per every molecule of NADH processed through ETC 2. The enzymes and electron carriers needed for the ETC are located outside the mitochondrial membrane (Step 6)

Answer 37

A

Question 38

``` A if only the first statement is true B if only the second statement is true C if both of the statements are true D if neither of the statements is true Regulation of the Citric Acid Cycle ``` 1. ETC carriers and enzymes are organized into four distinct protein complexes and two mobile carriers 2. Two mobile electron carriers are not tightly associated to any complexes

Answer 38

C

Question 39

Four Protein Complexes tightly bound to membrane: NADH-coenzyme Q reductase: Largest complex out of the 4 NADH from citric acid cycle is the source of electrons for this complex Contains >40 subunits including flavin mononucleotide (FMN) and several iron-sulfur protein clusters (FeSP) FMN: reactant for oxidation-reduction process; cofactor or reaction for redox reaction

Answer 39

Complex I Net result: Facilitates transfer of electrons from NADH to coenzyme Q Single step direct transfer (NADH—>coenzyme Q ) Oxidation reaction: NADH —> NAD (freeing the 2 electrons and 2H ions) 2e- and 2H reacts with FMN, reduce it forming FMNH2 Several intermediate reactions are involved in this electron transfer

Question 40

Four Protein Complexes tightly bound to membrane: Succinate-coenzyme Q reductase: Smaller than complex I Contains only four (4) subunits including two iron-sulfur protein clusters (FeSP) Succinate is converted to fumarate by this complex (Stage 6) In the process it generates FADH2

Answer 40

Complex II CoQ is the final recipient of the electrons from FADH2 Oxidized FADH2 = 2 H ions goes to CoQ Complex I and II common product is reduced coenzyme Q (CoQH2)

Question 41

Four Protein Complexes tightly bound to membrane: Coenzyme Q-cytochrome C reductase contains 11 different subunits Several iron-sulfur proteins (FeSP) and cytochromes are electron carriers in this complex Cytochrome is a heme iron protein in which reversible oxidation of an iron atom occurs Heme: present in hemoglobin and myoglobin Iron-sulfur protein will be reduced forming iron II sulfur proteins

Answer 41

Complex III Various cytochromes (cyt a, cyt b, cyt c) differ from each other by: Protein constituents The manner in which the heme is bonded to the protein Attachments to the heme ring Cytochrome C: common in humans CoQH2 (oxidized) —> CoQ + 2H+ + 2e- Electrons and H ions would be utilized by iron-sulfur proteins

Question 42

Four Protein Complexes tightly bound to membrane: Cytochrome C oxidase: final stage of electron transfer Contains 13 subunits including two cytochromes The electrons flow from ‘cyt c’ to ‘cyt a’ to ‘cyt a3’ The electrons from ‘cyt a3’ and hydrogen ion (H+) combine with oxygen (O2) to form water (H2O)

Answer 42

Complex IV (Cytochrome C oxidase) O2 + 4H+ +4e- —> 2H2O and ATP after the process It is estimated that 95 % of the oxygen used by cells serves as the final electron acceptor for the ETC