biochem lect 5 ch 16 citric acid cycle class

7/28/2019 Biochem Lect 5 Ch 16 Citric Acid Cycle Class

1/25

1

The Citric Acid Cycle

Michael Borenstein, Ph.D.
http://images.google.com/imgres?imgurl=http://api.ning.com/files/vw9KsAuQbHFrV6FYrcJT1wfPDUbEJ0LUMbZx0T4fZb*GaZC6o6Q9YqBFBuudjonGwQM1IOvW8R-9sEvF4wgx3t6pQxJ94nlt/ttar_orange_01_h_launch.jpg&imgrefurl=http://ocequality.ning.com/&usg=__bx2ShCdxeKLFgGkFPfmcAVcTA0Y=&h=335&w=310&sz=23&hl=en&start=3&tbnid=Bh5r0sYUAsGoGM:&tbnh=119&tbnw=110&prev=/images%3Fq%3Dorange%26gbv%3D2%26hl%3Den


2/25

2

Cellular respiration and energy production

- aerobic phase of catabolism comprising processes by

which cells consume O2 and produce CO2 and energy =cellular respiration

- comprises three stages:

1. oxidation of organic fuels to acetylCoA

2. conversion of acetylCoA to CO2 and reduced

forms of electron carriers NADH, FADH2

3. oxidation of electron carriers with O2 (final

electron acceptor) with release of energy,

converted to ATP (oxidative phosphorylation)


3/25

3

Cellular respiration and energy production

- carried out in mitochondria:


4/25

4

- PDH complex essentially converts pyruvate (3C) to acetyl Co-A (2C), yielding CO2and NADH; it has:

3 Enzymes5 Cofactors: Thiamine Pyrophosphate (TPP), Lipoic Acid, NAD+, FAD,

Coenzyme A (CoA-SH)(note that four different vitamins are involved: thiamine (B1), riboflavin (B2), niacin (B3), and pantothenate (B5))

It is an irreversible process !!

Oxidation of pyruvate to acetylCoA

- oxidative decarboxylation catalyzed by a cluster of enzymes and their cofactors =

pyruvate dehydrogenase complex (PDH):


5/25

5

Coenzyme A and acetylCoA

- structure: 3-phosphoadenosinediphosphate + pantothenic acid + mercaptoethanolamine:

- acetylCoA is more energy-rich than ATP:


6/25

6

Lipoic acid: key cofactor

- Lipoic acid (acting attached on Lys sidechain of dihydrolipoyl transacetylase) can

undergo redox reactions and can attach acetyl groups (similar to CoA):


7/25

7

(also called pyruvate

decarboxylase)

- PDH complex can be covalently modified by phosphorylation which makes it less active and

dephosphorylation to make it more active. These processes are catalyzed by PDH-kinase and PDH-phosphatase. An inherited decreased activity of PDH results in Chronic Lactic Acidosis (CLA).

Oxidation of pyruvate to acetylCoA- five consecutive steps, achieved in PDH complex:


8/25

8

Oxidation of pyruvate to acetylCoA

- PDH complex: supramolecular enzymatic complex of nanometric dimensions

containing:E1pyruvate dehydrogenase (aka pyruvate decarboxylase)

E2 dihydrolipoyl transacetylase (20 trimers = 60 molecules)

E3 dihydrolipoyl dehydrogenase

- the swinging lipoyl arm of E2 (blue) can reach the active sites of E1 and E3 (enzymesare mechanically coupled!!!):


9/25

9

SNR

H3C

N

N

N

S

H3CO P O P O

OO

O O

HH3C

NH2

CH3C

O

C

O

O

SNR

H3C

C CO

OOH

H3C

SNR

H3C

CH3C OH

CO2

SNR

H3C

CH3C OH

H

E1 E1E1E1

Thiamine

SS(CH2)4C

OHNE2 Lys

Hydroxyethyl TPP

Electron "Sink"

Enz-Base

pyrophosphate

E1

E2

=decarboxylase

=transacetylase

Pyruvate

TPP

E1

Decarboxylation of pyruvate

- achieved using TPP (vit B1) as cofactor, which carries the product = acetaldehyde:

(see also the fate of pyruvate in glycolysis)


10/25

10

SNR

H3C

CH3C OH

HH

CH3C O

CoAHSCH3C O

S

H

CoA

Lys

SS

(CH2)4C

O

HNE2

SS(CH2)4C

OHNE2Lys

SS

(CH2)4C

O

HNE2

Lys

..

Free thiamine

released to accept another

pyruvate

E1

Acetyl-S-CoA

Oxidation of activated acetaldehyde to acetate

- the active acetaldehyde bound on TPP is then transferred to the lipoyl moiety of

dihydrolipoyl transacetylase (E2), where it undergoes an internal redox reaction:(contd)


11/25

11

2 C

4 C6C

6 C

6 C

5 C

4 C

4 C

4 C

4 CCitric acid cycle


12/25

12

Citric acid cycle


13/25

13

Analogous to

PDH Complex

Citric acid cycle


14/25

14

Membrane

Bound

Citric acid cycle


15/25

15

Citric acid cycle


16/25

16

4-6

6

66

46

36-38

We will use 3 ATP for each NADH and 2 ATP for each FADH2


17/25

17

TCA is amphibolic: anabolic and catabolic


18/25

18

(anaplerotic = to fill up (gr.)

(concentration of various intermediates in citric acid cycle is kept ~ constant

through anaplerotic reactions)


19/25

19

Anapleroticreaction:

Pyruvate

Carboxylaseand use of Biotin

as a 1 carbon

carrier.

Note: the protein

avidin (found in raw

egg whites) stronglybinds to biotin and

can cause biotin

deficiency. Dont eat

too many raw eggs !!!

Bi l i l T h


20/25

20

- carry 1 or 2 carbon intermediates from one active site to another:

Biological Tethers


21/25

21

Regulation of

TCA and metabolic flow


22/25

22

Fate of Citrate

Citrate Shuttle

S d F t f S i t


23/25

23

Sources and Fate of Succinate

Heme


24/25

24


25/25

25

Goals and Objectives

Upon completion of this series of lectures at minimum you should be able to answer

the following:What is cellular respiration, what are its stages, what are the

main intermediates

and enzymes involved? What is the ultimate electron acceptor in vivo, and which

compounds are used in vivo for energy storage?

Which are the main structural particularities of mitochondria and where are variousenzymes relevant for cellular respiration placed?

Which are the main steps, intermediates, and enzyme/enzyme complexes

involved in oxidation of pyruvate to acetyl-CoA, what are their characteristics?

What are the main steps, intermediates, and enzymes involved in the citric acid

cycle, what are their characteristics, how is it regulated, and by which chemical

entities, what decides the fate of citrate, pyruvate, succinate ?

At which steps in the conversion of a molecule of glucose through glycolysis,

coupled with cellular respiration do we consume/produce energy and what is the

overall balance?

Why do we state that the citric acid cycle is amphibolic, what are anaplerotic

reactions (with examples), what is their significance?

How is the citric acid coupled with other important physiologic processes?

biochem lect 5 ch 16 citric acid cycle class

Documents