Mcb 450 Exam Ii

Front 1

Aerobic Fate of Pyruvate

Back 1

- Enters mitochondrion to be oxidized into carbon dioxide and water
- Process involved in citric acid cycle and one preparatory reaction
- Accounts indirectly for most energy (ATP) produced in cells

Front 2

Pyruvate Dehydrogenase Complex

Back 2

- Pyruvate + NAD+ + CoASH to AcCoA + CO2 + NADH + H+
- In mitochondrion
- Large multienzyme complex, can be seen in electron microscope

Front 3

PyrDehy Step I

Back 3

Decarboxylation
- Requires Thiamine pyrophosphate, TPP.
- TPP typically involved in decarboxylation of alpha ketoacids

Front 4

PyrDehy Step II

Back 4

Oxidation and Acyl Transfer
- Requires lipoic acid
- Lipoic acid in reduced for has 2 SH groups
- In oxidized form, there is a disulfide bond.

Front 5

PyrDehy Step III

Back 5

Acyl Transfer
- Acetyl group is transferred from reduced lipoic acid to Coenzyme A
- CoA forms thioester bonds with acyl groups and makes those groups both good nucleophiles and good electrophiles
- CoA derivatives are typically involved in carbon-carbon condensations

Front 6

PyrDehy Step IV

Back 6

Re-oxidation of reduced lipoic acid by FAD, flavine adenine dinucleotide
- FAD is an intermediate oxidizing agent in which two hydrogens are accepted as hydrogen free radicals
- The reduced for is FADH2
- Generally FAD is a stronger oxidizing agent that NAD+, but this instance is an exception. The protein modifies the oxidation potential of FAD.

Front 7

Properties of the TCA

Back 7

- Tricarboxylic acid cycle is the major site of carbon dioxide production in the body
- Occurs in mitochondrial matrix
- All enzymes except one are soluble
- Common to oxidation of sugars and fatty acids
- Requires oxygen

Front 8

Outline of TCA

Back 8

- 2 + 4 = 6
- 6 - 1 = 5
- 5 - 1 = 4, but not the same as original 4. However, it can be converted to original.
- See notes for illustration

Front 9

Citrate Synthase

Back 9

- OAA +AcCoA = Citrate + CoA
- CoA derivatives are both good nucleophiles and electrophiles
- In this instance AcCoA makes a nucleophilic attack on the keto group of OAA (oxaloacetate)

Front 10

Tricarboxylate Side

Back 10

- Conversion of a alpha-ketoglutarate to succinyl CoA involves the same mechanism as the pyruvate dehydrogenase mechanism
Cofactors:
- TPP to decarboxylate
- Lipoic Acid to oxidize and serve as an acyl group acceptor
- CoA to form a thioester
- FAD to reoxidize reduced lipoate
- NAD+ to reoxidize FADH2

Front 11

Dicarboxylate Side

Back 11

- Succinate to Fumerate to Malate to OAA
- In this sequence, a methylene group is converted to a keto group.
Steps:
- Forming a double bond
- Adding water across the double bond
- Oxidizing the -OH group to a keto group
- The same steps occur in oxidation of fatty acids
- See lecture for illustration

Front 12

Stoichiometry of TCA

Back 12

In each turn of TCA:
- 2 carbons enter as the acetyl groups of AcCoA
- 2 carbons are evolved as CO2
- 3 NADH + 3H+ are generated
- One FADH2 is produced
- One GTP is produced

Front 13

Glyoxylate Cycle (Plants)

Back 13

- In animal cells there can be no net synthesis of OAA from AcCoA
- For every 2 C atoms that enter the cycle, 2 are lost as CO2
- The best one can do is to regain the OAA which was used to condense with AcCoA
- Plants can get a net synthesis because they contain two enzymes that bypass the decarboxylation steps of TCA

Front 14

TCA Regulation

Back 14

-One known allosteric effect is ADP, which activates isocitrate dehydrogenase
- Pyruvate to AcCoA ('prep') is regulated by covalent modification Pyruvate Dehrydrogenase

Front 15

Covalent Modification of PyrDehy

Back 15

Enzyme has 2 forms:
- Enz-Ser-OH more active
- Enz-Ser-P less active
- These 2 are interconverted by a kinase and a phophatase