Mitochondrial carriers and substrate shuttles; Pentose phosphate pathway- Lecture 64

Question 1

Name the mitochondrial carriers.

Answer 1

1. phosphate carrier
2. dicarboxylate carrier
3. tricarboxylate carrier
4. alpha-Kg carrier
5. pyruvate carrier
6. glutamate carrier
7. aspartate carrier
8. adenine nucleotide carrier

Question 2

What compounds do not have carriers?

Answer 2

OAA, alpha GP, DHAP, NADH/NADPH

Question 3

What does mitochondrial carrier 1 exchange?

Answer 3

phosphate for OH

Question 4

What does mitochondrial carrier 2 exchange?

Answer 4

phosphate or malate or succinate for each other

Question 5

What does mitochondrial carrier 3 exchange?

Answer 5

citrate, isocitrate, malate or PEP for each other

Question 6

What does mitochondrial carrier 4 exchange?

Answer 6

alpha- KG for malate

Question 7

What does mitochondrial carrier 5 exchange?

Answer 7

pyruvate for OH or ketone bodies

Question 8

What does mitochondrial carrier 6 exchange?

Answer 8

glutamate for OH

Question 9

What does mitochondrial carrier 7 exchange?

Answer 9

aspartate for glutamate

Question 10

What does mitochondrial carrier 8 exchange?

Answer 10

ADP for ATP

Question 11

How do mitochondrial carriers 1 and 8 work together?

Answer 11

8 transports ATP produced in oxidative phosphorylation out of the mito in exchange for cytosolic ADP (produced from ATP hydrolysis in to the cytosol) while carrier 1 brings ATP plus Pi back to the mito for ATP synthesis

Question 12

Which carriers can malate be exchanged on?

Answer 12

2, 3, and 4

Question 13

Compounds which cannot enter or leave the mito because they lack carriers have to be _______.

Answer 13

transported by substrate shuttle mechanisms.

Question 14

Since NADH/NAD+ and NADPH cannot enter or leave the mito, what must be transported instead?

Answer 14

reducing equivalents (hydrogen and electrons)

Question 15

What are shuttles critical for?

Answer 15

transporting reducing equivalents from NADH or NADPH into or out of the mito
providing acetyl CoA for fatty acid or cholesterol synthesis
providing carbon intermediates for gluconeogenesis

Question 16

What are the major shuttles for translocating the reducing equivalents of NADH into the mito?

Answer 16

alpha-glycerophosphate (alpha-GP or glycerol 3P)

malate-aspartate (MA) shuttle

Question 17

What is transamination?

Answer 17

the transfer of an alpha amino group of one amino acid to a keto acceptor to produce a new amino acid and a new keto acceptor (part of the MA)

Question 18

Describe the alpha glycerophosphate shuttle.

Answer 18

DHAP (product of glycolysis) reacts with NADH to produce alphaGP and NAD+ via cytosolic alphaGPDH
need to regenerate DHAP to continue with glycolysis –> alphaGP reacts with outer surface
of mitochondria membrane bound alphaGPDH which also initiates E-FAD –> E-FADH reaction in the membrane (which is regenerated via Q –> QH reaction) which can donate those reducing equivalents to the respiratory chain

Question 19

Describe the MA shuttle.

Answer 19

Glycolysis produces NADH which reacts with OAA to regenerate NAD+ and malate (via cytosolic malate dehydrogenase)
malate is transported into the mito viacarriers 2,3, or 4
malate in the mito produces NADH and OAA (via mitochondrial malate dehydrogenase)
OAA must be converted to aspartate to exit the mito via carrier 7 (which is done by converting it from glutamate via aspartate amino transferase with a secondary product of alpha-ketoglutarate)
alphaKG can exit via carrier 4 and, once outside, the reaction can be reversed to produce glutamate (which can reenter the mito via carrier 7) and OAA

Question 20

How can you differentiate between the two mitochondrial shuttle mechanisms?

Answer 20

alphaGP makes FADH while MA makes NADH
alpha GP does not cross mito membrane (could inhibit carriers and only affect MA)
different enzymes are used for each

Question 21

Describe the flow of the isocitrate shuttle.

Answer 21

reducing equivalents of NADPH are used to synthesize isocitrate from alphaKG (via mito NADPH-linked isocitrate dehydrogenase)
isocitrate leaves the mito on carrier 3
isocitrate reacts with cytosolic NADP-linked ICDH to produce NADPH plus CO2 and alphaKG
alphaKG enters mito on carrier 4

Question 22

Where does the PPP pathway occur?

Answer 22

the cytosol of liver, adipose, mammary glands, steroidogenic tissues (eg. adrenals), and RBCs
low activity in the brain, muscle, and heart

Question 23

What does the PPP pathway do?

Answer 23

uses glucose to:
produce NADPH (needed for synthesis of cholesterol, fatty acids, steroids, detoxification reactions by the cytochrome P450 system)
production of ribose (needed to produce the nucleotides for RNA adn DNA synthesis and for NAD, FAD, CoASH)
reduction of oxidative stress

Question 24

Describe the oxidative steps of the PPP pathway.

Answer 24

1. glucose-6P + NADP+ –> 6-phosphogluconic acid + NADPH (via G6P dehydrogenase)
2. 6-phosphogluconic acid+ NADP+ –> CO2 + ribulose 5-P + NADPH (via 6-phosphogluconic acid dehydrogenase)

Question 25

List the important intermediates of the non-oxidative steps of the PPP pathway.

Answer 25

Ribulose 5P Xyulose-5P sedoheptulose-7P erythrose-4P

Question 26

List the important enzymes involved in the non-oxidative steps of the PPP pathway.

Answer 26

phosphopentoisomerase phosphopentoepimerase transketolase (thiamine pyrophosphate cofactor) transaldolase

Question 27

What is the function of the non-oxidative PPP pathway?

Answer 27

convert pentoses into compounds we can use metabolically

Question 28

What is the net result of the PPP pathway?

Answer 28

6 NADP+ +0.5G6P --> 3 CO2 + 6 NADPH

Question 29

What regulates the PPP pathway?

Answer 29

availability of G6P and need for NADPH (NADPH/NADP ratio)

Question 30

____ is made in aerobic cells from ______. This means that RBCs are ______ because ______.

Answer 30

Hydrogen peroxide (H2O2) mito or autooxidation of ferrous hemes like hemoglobin always in oxidative stress H2O2 is a powerful oxidant

Question 31

How do RBCs decompose H2O2?

Answer 31

``` catalase (heme-containing enzyme that decomposes H2O2 to O2 + H2O) glutathione peroxide (uses gluthione to catalyze H2O2 + 2 GSH --> 2 H2O + GSSG) ```

Question 32

How do RBCs regenerate GSH from GSSG?

Answer 32

GSSG + 2 NADPH --> 2 GSH + 2 NADP+ (via gluthione reductase)

Question 33

What is G6PDH deficiency?

Answer 33

most common inborn error of metabolism severity determined by how much G6PDH activity is present (complete deficiency is embryonically lethal) individuals are sensitive to oxidative stress (via drugs, foods, chemicals, high O2, high altitude) resulting in hemolytic anemia due to inability to efficiently clear H2O2 from RBCS