Glycolysis and the TCA Cycle

Question 1

how many molecs of pyruvate does glucose yield?

Answer 1

2 (pyruvate is a 3C compound)

Question 2

Where is glycolysis the main source of energy?

Answer 2

RBC (no mitochondria), sperm

Question 3

how much ATP is invested in glycolysis (preparatory phase)?

Answer 3

2 ATP

Question 4

Glucose transporters

Answer 4

GLUT4: insulin sensitive (skeletal muscle, adipose tissue)
Glut2: insulin independent (liver)

Question 5

Where does glycolysis occur?

Answer 5

cytoplasm

Question 6

Glycolysis results

Answer 6

net 2 ATP (2 invested, 4 made)

NADH

Question 7

First reaction of glycolysis where ATP is invested; catalyzed by…

Answer 7

Activation of glucose
-glucose to G6P (w/ ATP going to ADP)

Catalyzed by:
glucokinase (liver)
hexokinase (everywhere else)

Question 8

Hexokinase vs glucokinase

Answer 8

Hexokinase:
not very selective
present in all cells 
low Km for sugars
inhibited by G6P

Glucokinase (high Vmax):
selective for glucose
liver, pancreatic Beta cells
High Km for glucose
Inhibited by F6P

Question 9

Second ATP investment in glycolysis

Answer 9

F6P + ATP–>F1,6BP + ADP
Catalyzed by: PFK1 (phosphofructokinase 1)

**Rate limiting step
irreversible

F2,6BP and AMP encourage rxn
–F6P can be converted to F2,6BP by PFK2. F2,6BP is the most potent activator of PFK1
ATP, citrate discourage rxn

Question 10

Most potent activator of PFK1

Answer 10

F2,6BP

Question 11

PFK2

Answer 11

bifunctional:
kinase (adds phosphate)
or phosphatase (removes phosphate)

in liver:
In the fed state– (increases rate of glycolysis)
high insulin/glucagon ratio causes decreased cAMP and reduced levels of protein kinase A. This favors dephosphorylation (activation) of PFK2/FBP-2 complex. (Makes F2,6BP which activates PFK1)

During starvation, glucagon rises, insulin drops, and cAMP dependent PKA phosphorylates PFK2 (it becomes fructose 2,6 bisphosphatase which removes P from F2,6BP and becomes F6P again– promoting gluconeogenesis)

Question 12

First NADH generation and ATP generation

Answer 12

Glyceraldehyde 3 phosphate to 1,3BPG (NADH)

1,3 BPG to 2,3Phosphoglycerate +ATP

Question 13

2nd ATP generation

Answer 13

-substrate level phosphorylation
**imp rxn
2 phosphoenol pyruvate + 2ADP–> 2 pyruvate +2ATP

enzyme: pyruvate kinase (irreversible rxn)

stimulated by F1,6BP in glycolysis
Enz inhibited by ATP, alanine, PKA

In fasting (liver), glucagon dependent inactivation of pyruvate kinase via phosphorylation by PKA prevents glycolysis.
Dephosphorylation by phosphoprotein phosphatase results in reactivation of pyruvate kinase

Question 14

Fate of pyruvate

Answer 14

Can become lactate, or can enter TCA cycle

Fed state:

• converted to alanine an aa for protein synthesis
• excess carb intake can lead to increased production of pyruvate which can enter mitochondria, increasing amnt of acetyl coA available for fatty acid synthesis

In fasting state:
-pyruvate coming from lactate is converted to oxaloacetate by pyruvate carboxylase), providing carbon skeletons for gluconeogenesis

Question 15

Enzyme converting pyruvate to acetyl CoA

and coenzymes

Answer 15

pyruvate dehydrogenase***
-located in mitochondrial matrix
-Coenzymes:
coenzyme A
thiamine pyrophosphate (TPP)
prosthetic groups
flavin adenine dinucleotide (FAD)
nicotinamide adenine dinucleotide (NAD)
Lipoate

Vitamins are part of the cofactors:
thiamine (B12) TPP
riboflavin (B2) FAD
niacin NAD
pantothenate coenzyme A

Kinase and phosphatase are also part of this enzyme complex

Question 16

Wernicke’s Encephalopathy

Answer 16

-thiamine deficiency
-inability to oxidize pyruvate (critical fuel for the brain)
-altered mental status and cranial nerve palsies
Dx: high levels of pyruvate in blood

Thiamine deficiency can also present as heart failure (Beriberi) bc glucose is an imp fuel for the heart.

Question 17

Regulation of PDH

Answer 17

• allosteric reg by feedback inhibition from ATP, acetyl CoA, NADH and fatty acids
• AMP, CoA, and NAD+ allosterically activate it

Fasting state:

• PHD is in inactive phosphorylated state (the kinase that does this is inhibited by pyruvate and stimulated by ATP)
• in liver, inhibition of PDH ensures pyruvate can be redirected to gluconeogenesis

Fed state:
PDH is active in dephospho state (insulin and ADP are high)

Ca stimulates phosphatase which dephosphorylates PDH to become active

Question 18

Why is acetyl CoA an important hub of metab?

Answer 18

carbon skeletons of sugar, fatty acids, and several aa enter TCA cycle as acetyl CoA

Question 19

Where does TCA cycle take place?

Answer 19

mitochondrial matrix

Question 20

What is generated in the TCA cycle?

Answer 20

3 NADH
FADH2
GTP
2 CO2

Question 21

First rxn of TCA cycle

Answer 21

condensation rxn: acetyl CoA (2C) and oxaloacetate (4C) to yield citrate (6C)
Enzyme: citrate synthase

Citrate = feedback inhibitor of PFK1 in glycolysis (F6P to F1,6BP)
Citrate can also leave TCA cycle to form fatty acids (lipogenesis)

Question 22

TCA and fasting

Answer 22

gluconeogenic precursors are converted to malate, which leaves the mitochondiria to enter the pathway of gluconeogenesis in cytosol

Question 23

alpha ketoglutarate

Answer 23

imp entrance point for many amino acids that contribute to gluconeogenesis

Question 24

succinyl coa

Answer 24

imp entrance for aa and for breakdown products of fatty acids with an odd number of carbons that contribute to gluconeogenesis

Question 25

fumarate

Answer 25

important entrance for aa and it is a byproduct of urea cycle

Question 26

oxaloacetate

Answer 26

involved in gluconeogenic pathway from pyruvate

Question 27

substrate level phosphorylation in TCA

Answer 27

GTP formation through succinyl CoA to succinate