Chapter 18

Question 1

Glycolysis and the Citric Acid Cycle

Answer 1

• Glycolysis: glucose to pyruvate
• occurs in cytoplasm
• Citric acid cycle: acetyl entry (as acetyl-CoA)
• occurs in mitochondria matrix

Question 2

Acetyl CoA

Answer 2

• Made from pyruvate under aerobic conditions
• Reaction occurs in the mitochondria
• Serves as fuel for the citric acid cycle
• The two-carbon acetyl unit processed to two molecules of CO2
• Generates high-energy electrons that can be used to form ATP

Question 3

Pyruvate Dehydrogenase Complex

Answer 3

• mitochondrial matrix enzyme
• oxidatively decarboxylates pyruvate to form acetyl CoA
• links glycolysis and citric acid cycle (CAC)
• Irreversible enzyme complex with 3 enzyme activities
  1. Oxidative decarboxylation
  2. Transacetylase
  3. Enzyme re-oxidation

Question 4

PDH Structure/Function

Answer 4

• 3 components with 4 main reactions
  E1: pyruvate dehydrogenase component
  E2: dihydrolipoyl transacetylase component
  E3: dihydrolipoamide dehydrogenase component

Question 5

E1 pyruvate dehydrogenase component

Answer 5

1. Decarboxylation
   - Lose CO2 when attaching pyruvate to TPP
2. Oxidation
   - As hydroxy ethyl is removed from TPP by lipoamide from E2

Question 6

E2 dihydrolipoyl transacetylase component

Answer 6

3. Oxidation
   - Lipoamide swings to bring acetyl into E2
4. Transfer acetyl to CoA

Question 7

E3 dihydrolipoamide dehydrogenase component

Answer 7

5. Reoxidation of enzyme

Question 8

Overview of Pyruvate Dehydrogenase Reactions

Answer 8

• Requires 3-enzyme components with 4 main reactions:
  1. Decarboxylation
  2. Oxidation
  3. Transfer to CoA
  4. Re-oxidation
• Requires 5 coenzymes: thiamine pyrophosphate, lipoic acid, FAD, CoA, NAD+

Question 9

Close up of Decarboxylation: E1

Answer 9

• E1 (pyruvate dehydrogenase component) catalyzes the decarboxylation
• Pyruvate combines with ionized form of thiamine pyrophosphate (TPP) and is decarboxylated (CO2
  is lost) to generate hydroxyethyl-TPP
• TPP is a first coenzyme (thiamin is also known as vitamin B1)

Question 10

Close up of Oxidation: E1

Answer 10

• E1 (pyruvate dehydrogenase component) provides hydroxyethyl-TPP to lipoamide
  of E2 (dihydrolipoyl transacetylase component)
• As hydroxyethyl is transferred to lipoamide of E2 it is oxidized to an acetyl
  
  • the disulfide group of lipoamide is reduced to its disulfhydryl form
• yields acetyl–lipoamide.
• Lipoic acid is a second coenzyme

Question 11

Close up of Transfer to CoA: E2

Answer 11

• E2 (dihydrolipoyl transacetylase component) transfers the acetyl group from acetyl-lipoamide to coenzyme A
• Results in Acetyl CoA + dihydrolipoamide
• Coenzyme A is a third coenzyme (pantothenate of CoA is also known as vitamin B5)

Question 12

Close up of Reoxidation: E3 - part 1

Answer 12

• E3 (dihydrolipoamide dehydrogenase) oxidizes dihydrolipoamide to lipoamide using FAD component so that it can oxidize another hydroxyethyl-TPP
• FADH2 in E3 must also be oxidized back to FAD by NAD+

Question 13

Close up of Reoxidation: E3 - part 2

Answer 13

• Requires FAD and NAD+ (4th and 5th coenzymes)
• Riboflavin in FAD is also known as vitamin B2
• Niacin in NAD+ is also known as vitamin B3
• FAD is in the PDH complex structure
• Increases the electron transfer potential of FADH2
• FADH2 electrons can be transferred to NAD+
• Normally NADH + H+ transfers to FAD

Question 14

Key Irreversible Step

Answer 14

• acetyl CoA forms from pyruvate pyruvate dehydrogenase is a key irreversible step

Question 15

Acetyl CoA has two principle fates:

Answer 15

1. metabolism by the citric acid cycle
   - usually to support energy production
2. incorporation into fatty acids (i.e., lipid
   synthesis)
   - store energy, usually from excess carbohydrates

Question 16

Regulation of Pyruvate Dehydrogenase

Answer 16

• main response is based on energy needs
• Lots of energy → inhibit PDH
• Low energy → activate PDH
• Mainly by covalent modification of E1
• Inactivated by PDH kinase (phosphorylation)
• ATP, NADH & acetyl CoA activate the kinase (inhibit PDH)
• ADP & pyruvate activate inhibit the kinase (activate PDH)
• Activated by PDH phosphatase
• Ca2+ activates phosphatase (activate PDH)
• From contractions in muscle
• From epinephrine signal in liver
• Insulin activates phosphatase (activate PDH)
• Fed state/energy storage mechanism
• Only in tissues that make fatty acids (ex. liver and
  adipose)

Question 17

Clinical Insight: Beriberi and TPP

Answer 17

• Beriberi is a disease caused by a vitamin deficiency
• Lack of thiamine in diet
• Not able to make thiamine pyrophosphate (TPP)
• TPP is a coenzyme several enzymes: pyruvate dehydrogenase complex (PDH), α-ketoglutarate dehydrogenase (CAC cycle), transketolase (pentose phosphate pathway)
• More prevalent in populations dependent on rice consumption
• Rice is low in thiamine
• Polishing of rice increases storage life but decreases thiamine content
• Rice is high in carbohydrate
• Increased demand for TTP enzymes