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BIOCH 310 Part 1 > Lecture 5 Production of Acetyl-CoA > Flashcards

Lecture 5 Production of Acetyl-CoA Flashcards

1
Q

What is cellular respiration?

A

Catabolism of substrates which are oxidized to CO2 and H2O, in the presence of oxygen.

  • A set of metabolic reactions and processes that take place in the cellular mitochondria to convert chemical energy from oxygen molecules or nutrients into adenosine triphosphate, and then release waste products.
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2
Q

Range of carbon oxidation states

A

Carbon can exhibit oxidation states ranging from

-4 (methane: CH4) to 0 (glucose: (CH2O)6) to +4 (carbon dioxide: CO2)

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3
Q

What oxidation happens to carbon and oxygen during cellular respiration?

A
  • carbon atoms are oxidized to CO2 (oxidation state = +4)
  • molecular oxygen (oxidation state = 0) is reduced to H2O (oxidation state of oxygen = -2).
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4
Q

Stages of cellular respiration

A

Stage 1: Formation of acetyl-CoA from glucose, fatty acids and some amino acids.

Stage2: Oxidation of acetyl-CoA by the citric acid cycle and generation of NADH and FADH2.

Stage 3: Oxidation of the NADH and FADH2 by the electron transfer chain coupled to ATP synthesis.

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5
Q

Where does pyruvate form into AcetylCo-A?

A

Occurs in the mitochondrial matrix

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6
Q

Anatomy of the mitochondria

A
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7
Q

What are the 4 compartments of the mitochondrial matrix and role in cellular respiration?

A
  • Outer membrane → Has transmembrane channels called porins which are permeable to molecules <5 kDa
  • Inner membrane → essentially impermeable so selective and requires transporters; ETC enzymes found here
  • Intermembrane space → between outer and inner membrane; contains cytochrome c
  • The matrix → inside inner membrane; contains many oxidative enzymes systems such as the PDH complex and citric acid cycle enzymes
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8
Q

How does pyruvate move from the cytosol into the mitochondrial matrix?

A
  1. diffuses through the outer membrane via porin
  2. Transported through the inner membrane with a proton via pyruvate translocase
  3. Once in the matrix, the pyruvate dehydrogenase complex oxidizes pyruvate to acetyl-Coenzyme A
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9
Q

Reaction of pyruvate → acetyl-CoA

A

Oxidative decarboxylation via pyruvate dehydrogenase

  • practically irreversible
  • exergonic → ∆G’o= -33.4 kJ mol-1
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10
Q

Structure of AcetylCo-A

A

Acetyl-CoA is an acetyl group in thioester bond with coenzyme A.

  • Coenzyme A contains pantothenic acid (vitamin B5) and mercaptoethylamine whose sulfhydride is in thioester bond with acetate.
  • Also contain an ADP
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11
Q

What part of AcetylCo-A makes it high energy?

A

The thioester linkage is a “high energy” bond, which is particularly reactive.

  • Hydrolysis of this thioester bond is exergonic.
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12
Q

What does Coenzyme A function in?

A

Acyl group transfer reactions.

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13
Q

What is the PDH complex composed of?

A

Pyruvate dehydrogenase

3 enzymes (E1, E2, E3) and requires 5 cofactors.

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14
Q

What are the 3 enzymes of PDH?

A
  • E1: pyruvate dehydrogenase
  • E2: dihydrolipoyl transacetylase
  • E3: dihydrolipoyl dehydrogenase
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15
Q

What are the cofactors of PDH and their location and function?

A
  • thiamine pyrophosphate (TPP)
  • lipoic acid
  • coenzyme A(CoA)
  • flavin adenine dinucleotide (FAD)
  • nicotinamide adenine dinucleotide (NAD+)
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16
Q

Steps of oxidative decarboxylation of pyruvate by the PDH complex

A
  1. CO2 is removed and a hydroxyethyl group is attached to the TPP cofactor of E1. This is the slowest step and limits the overall rate of the reaction. This reaction is identical to the pyruvate decarboxylase reaction.
  2. The hydroxyethyl group is oxidized to an acetyl group and the disulfide bond in lipoamide is reduced. The acetyl group is transferred to the lipoamide in a thioester linkage on E2. The energy of oxidation is conserved in the thioester linkage.
  3. The lipoamide transfers the acetyl group to CoA, forming acetyl- CoA. The TPP on E1 is 45-60 Å from the FAD on E3. Multiple swinging arms likely contribute to bridging this distance.
  4. A series of redox events take place: the reduced lipoamide is re- oxidized by FAD which is reduced to FADH2 on E3. Note that FAD is a prosthetic group on E3; thus being regenerated in place.
  5. FADH2 is oxidized to FAD, as NAD+ is reduced to NADH.
17
Q

What cofactors are regnerated within the actual PDH complex?

A

The ones actually attached to the complex including TPP, lipoamide and FAD

18
Q

Why are multienzyme complexes more efficient?

A
  • The distance between active sites is short → the distances for products to diffuse is short which increases reaction velocity.
  • Side reactions are unlikely as reaction intermediates are channeled from one enzymatic subunit to another without leaving the multienzyme complex. In systems of isolated enzymes, intermediates are often lost through interactions with non-relevant enzymes and reactants.
  • The complex is controlled as one unit.
19
Q

What can acetylCoA be made from?

A
  • pyruvate (through glycolysis)
  • fatty acids (through beta oxidation)
  • ketogenic amino acids
  • ketones
20
Q

Can animals make glucose from fat?

A

No because although fat can be biosynthesized to Acetyl-CoA, you cannot make oxaloacetate directly from Acetyl-CoA and fat cannot be biosynthesized to oxaloacetate.

  • plants and microorganisms can however via the glyoxylate cycle
21
Q

How is the PDH complex regulated?

A
  • Allosterism
  • covalent modification via phosphorylation
22
Q

What inhibits PDH allosterically?

A

PDH is inhibited by:

  • End-products → acetyl-CoA and NADH.
  • Energy surplus (high ATP/ADP, high NADH/NAD+).
23
Q

What activates PDH allosterically?

A

PDH is activated by:

  • CoA (substrate)
  • Lack of energy (low NADH/NAD+), i.e., when energy demands are high
24
Q

How is PDH inhibited by covalent modification?

A

PDH is inhibited by the phosphorylation of E1

  • ATP-mediated phosphorylation of E1 by pyruvate dehydrogenase kinase.
    • Pyruvate dehydrogenase kinase is activated when [ATP] / [ADP] is elevated
    • as well as by allosteric end-product negative feedback = when the products of the PDH reaction, acetyl-CoA and NADH, are elevated
25
Q

How is PDH activated by covalent modification?

A

PDH is activated by:

  • The dephosphorylation of phosphorylated E1 by pyruvate dehydrogenase phosphatase.

BIOCH 310 Part 1 (16 decks)

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