Chem/Biol 472 W 2011 First Exam

0123

0 11 22 33 44 55 66 77 88 99

score

Fre

qu

ency

Frequency

Average = 76.4 = B-

A = 96+A-=90-95

B+ = 85-89B= 77-84

B- = 71-76C+ = 65-70

C= 55-64

CHAPTER 16 The Citric Acid Cycle

– Cellular respiration– Conversion of pyruvate to activated acetate – Reactions of the citric acid cycle– Regulation of the citric acid cycle– Conversion of acetate to carbohydrate precursors

in the glyoxylate cycle

Key topics:

Only a Small Amount of Energy Available in Glucose is Captured

in Glycolysis

2G’° = -146 kJ/mol

Glycolysis

Full oxidation (+ 6 O2)

G’° = -2,840 kJ/mol6 CO2 + 6 H2O

GLUCOSE

Cellular Respiration

• process in which cells consume O2 and produce CO2

• provides more energy (ATP) from glucose than glycolysis

• also captures energy stored in lipids and amino acids

• evolutionary origin: developed about 2.5 billion years ago

• used by animals, plants, and many microorganisms

• occurs in three major stages:

- acetyl CoA production

- acetyl CoA oxidation

- electron transfer and oxidative phosphorylation

Respiration: Stage 1

Generates some:ATP, NADH, FADH2

Respiration: Stage 2

Generates more NADH, FADH2 and one GTP

Respiration: Stage 3

Makes lots of ATP

In Eukaryotes, Citric Acid Cycle Occurs in Mitochondria

• Glycolysis occurs in the cytoplasm• Citric acid cycle occurs in the mitochondrial matrix†

• Oxidative phosphorylation occurs in the inner membrane

† Except succinate dehydrogenase, which is located in the inner membrane

Sequence of Events in the Citric Acid Cycle

• Step 1: C-C bond formation to make citrate

• Step 2: Isomerization via dehydration, followed by

hydration

• Steps 3-4: Oxidative decarboxylations to give 2

NADH

• Step 5: Substrate-level phosphorylation to give GTP

• Step 6: Dehydrogenation to give reduced FADH2

• Step 7: Hydration

• Step 8: Dehydrogenation to give NADH

The Citrate Synthase Reaction

• The only cycle reaction with C-C bond formation

• Essentially irreversible process

Oxidation of -ketoglutarate

• Enzyme: -ketoglutarate dehydrogenase complex

• Similar to pyruvate dehydrogenase complex

• Same coenzymes, identical mechanisms

The succinyl-CoA synthetase reaction.

1. A phosphoryl group replaces the CoA of succinyl-CoA bound to the enzyme, forming a high-energy acyl phosphate.

2. The succinyl phosphate donates its phosphoryl group to a His residue of the enzyme, forming a high-energy phosphohistidyl enzyme.

3. The phosphoryl group is transferred from the His residue to the terminal phosphate of GDP (or ADP), forming GTP (or ATP).

This is a substrate-level phosphorylation. Name another one…

Phosphoglycerate kinase

This is a reversible reaction!!! What other NTP generating reaction is irreversible?

Animal cells have another isozyme that uses ATP

Nucleotide diphosphate kinase

• GTP + ADP ↔ ATP + GDP

ΔGo’ = 0

The succinyl-CoA synthetase reaction. Active site of succinyl-CoA synthetase of E. coli .

The active site includes part of both the α and the β subunits.

The power helices place the partial positive charges of their helix dipoles near the phosphate group of P–His246 in the α chain, stabilizing the phosphohistidyl enzyme.

.

Succinate Dehydrogenase

• Covalently bound FAD is reduced to FADH2

• FADH2 passes electrons to coenzyme Q

• Reduced coenzyme (QH2) can be used to make

ATP

Hydration of Fumarate to Malate

• Fumarase is highly stereospecific• OH- adds to fumarate …

then H+ adds to the carbanion• Net effect: trans addition of water• Reversible reaction

Not a substrate for this enzyme!Fumarase substrate

Oxidation of Malate to Oxaloacetate

• Thermodynamically unfavorable reaction • Oxidation occurs because oxaloacetate

concentration is very low as it is continuously used

to make citrate

Products from One Turn of the Cycle

Net Effect of the Citric Acid Cycle

Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2 H2O

2CO2 +3NADH + FADH2 + GTP + CoA + 3H+

• carbons of acetyl groups in acetyl-CoA are

oxidized to CO2

• electrons from this process reduce NAD+ and FAD

• one GTP is formed per cycle, this can be

converted to ATP

• intermediates in the cycle are not depleted

COO-CH3- +CoASH CH3

C=OCoASH

When in the TCA cycle would this label be lost as CO2?

Let's sing!!

• http://www.csulb.edu/~cohlberg/songbook.html

Lyrics

http://books.google.com/books?id=oq9ENyL_d9YC&printsec=frontcover&source=gbs_v2_summary_r&cad=0#v=onepage&q=&f=false

Role of the Citric Acid Cycle in Anabolism

Anaplerotic Reactions

• these reactions replenish metabolites for the cycle

• four carbon intermediates are formed by

carboxylation of three-carbon precursors

The role of biotin in the reaction catalyzed by pyruvate

carboxylase. Biotin is attached to the enzyme through an amide bond

with the ε-amino group of a Lys residue, forming biotinyl-enzyme.

Biotin-mediated carboxylation reactions occur in two phases,

generally catalyzed in separate active sites on the enzyme as

exemplified by the pyruvate carboxylase reaction.

The role of biotin in the reaction catalyzed by pyruvate carboxylase. Biotin-mediated carboxylation reactions occur in two phases, generally catalyzed in separate active sites on the enzyme as exemplified by the pyruvate carboxylase reaction. In the first phase (steps 1 to 3), bicarbonate is converted to the more activated CO2, and then used to carboxylate biotin.

The biotin acts as a carrier to transport the CO2

from one active site to another on an adjacent monomer of the tetrameric enzyme.

In the second phase (steps 5 to 7), catalyzed in this second active site, the CO2 reacts with pyruvate to form oxaloacetate.

Regulation of the Citric Acid Cycle

Role of citrate in glycolysis regulation?

Figure 21-25Regulation of the citric acid cycle.

Pag

e 79

1

Glyoxylate CycleIn plants and some other special creatures, Acetyl CoA can serve as a glucose precursor!

2 acetyl groups enter the cycle and four carbons leave as

succinate.

• LEHNINGER • PRINCIPLES OF BIOCHEMISTRY

• Fifth Edition

David L. Nelson and Michael M. Cox

© 2008 W. H. Freeman and Company

CHAPTER 17Fatty Acid Catabolism

“Alfonse, Biochemistry makes my head hurt!!”\