SYLLABUS 5: Mitochondrial Carriers and Substrate Shuttles: the Pentose Phosphate Pathway Flashcards Preview

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Flashcards in SYLLABUS 5: Mitochondrial Carriers and Substrate Shuttles: the Pentose Phosphate Pathway Deck (37):

purpose of mitochondrial carriers? 

transport metabolic substrates into and out of the mitochondria, b/c of the limited permeability of the mitochondrial membrane


carrier 1, phosphate, action

phosphate carrier:

exchanges Pi with OH


carrier 2, dicarboxylate, action

dicarboxylate carrier: 

exchanges Pi or malate or succinate for each other


carrier 3, tricarboxylate, action

tricarboxylate carrier: 

exchanges citrate, isocitrate, malate, or PEP for each other


carrier 4, aKg, action

aKg carrier: 

exchanges aKg for malate


carrier 5, pyruvate, action

pyruvate carrier: 

exchanges pyruvate for OH or ketone bodies


carrier 6, glutamate, action

glutamate carrier: 

exchanges gultamate for OH


carrier 7, aspartate, action

aspartate carrier: 

exchanges aspartate for glutamate


carrier 8, adenine nucleotide, action

adenine nucleotide carrier: 

exchanges ADP for ATP 


which carriers can malate exchange on?

2: dicarboxylate carrier 

3: tricarobxylate carrier 

4: aKg carrier


what kind of exchange do carriers catalyze

1:1 exchange w/ their partners


which carriers funciton in oxidative phosphorylation?

carriers 1 and 8 

carrier 1: exhanges Pi with OH - provides phosphate

carrier 8: exchanges ADP for ATP - provides ADp or ATP


what is the purpose of mito shuttles?

compounds that can't enter or leave mito b/c they lack carriers are directly transported by substrate shuttle mechanisms 



what happens if NADH produced in G3PDH of glycolysis isn't reoxidized to NAD+?

glycolysis stops because of lack fo NAD+ 

however, NADH cannot enter or leave mito 

thus reducing equivalents of ANDH (electrons, hydrogens) are transported into mito by shuttles 


specific functions of shuttles?

1) transport reducing equivalents from NADH or NADPH into or out of the mito 

2) provide acetyl CoA for fatty acid or cholesterol synthesis 

3) provide carbon intermediates for gluconeogenesis


what shuttles translocate the reducing equivalents of NADH into the mito?

1) a-glycerophosphate shuttle

2) malate-asparata shuttle


what is the purpose of the alpha-glycerophosphate shuttle?

carries 2 electrons of NADH, aka reducing equivalents, from the cytosol into the mitochondrial matrix, using cytosolic glycerophosphate dehydrogenase


how does the a-glycerophosphate shuttle work?

DHAP reacts with NADH [both from glycolysis] to produce a-GP [glycerol 3 phosphate] + NAD+; catalyzed by cytosolic a-GPDH 

NAD+ is thus reoxidized 

now, must regenerate DHAP and do something w/ a-GP

a-GP reacts w/ mitochondrial a-GPDH, on the outer surface of the mito inner membrane to regenerate DHAP 

mito a-GPDH is linked w/ FAD, so FADH2 is also produced

FADH2 reacts w/ Q of the ETC to make QH2 which carries the H's to the ETC


what is the movement of reducing equivalents in the a-glycerophosphate shuttle

cytosolic NADH's reducing equivalents were transferred to DHAP to produce a-GP 

a-GP gave those reducing equivalents to mito FAD to produce FADH​2

FADH2 gives those H's to the respiratory chain 


do DHAP or a-GP enter the mito during the a-glycerophosphate shuttle?

no, because they don't have carriers


what's the purpose of the malate-aspartate shuttle?

to reoxidize the NADH from glycolysis, in the cytosol, or to regenerate OAA 


what is the process of the malata-aspartate shuttle


NADH from glycolysis transfers H's to OAA

OAA is reduced by Malate by cytosolic malate DH 

Malate enters Mito via carriers 2, 3, 4 


Malate reacts w/ Mito Malate DH, becomes OAA + Mitochondrial NADH 

These H's from NADH will -> e- transport chain 

OAA undergoes transamination reaction w/ glutamate to make aspartate; glutamate becomes a-KG 

Aspartate leaves mito via Carrier 7, a-Kg leaves via Carrier 4 


Reverse transamination reaction occurs: a-Kg reacts w/ Aspartate, regenerate Glutamate from a-Kg and OAA from Aspartate



purpose of the isocitrate shuttle?

transfer the reducing equivalents of mitochondrial NADPH to cytosolic NADP to make cytosolic NADPH 

transhydrogenase enzyme does this 

need to do this b/c NADPH is needed for biosynthetic reactiosn that occur largely in the cytosol 


how does the isocitrate shuttle work?

NADPH in the mitochondrial matrix reacts with a-KG and CO2 and the mitochondrial isocitrate DH to produce NADP+ and isocitrate 

isocitrate leaves the mito via carrier 3 

in the cytosol, cytosolic isocitrate DH acts on isocitrate and NADP+ to make NADPH and a-KG and CO2 

a-KG returns to the mito via carrier 4



where does the pentose phosphate pathway occur? 

cytosol of certain tisses - the liver, adipose, mamary gland, steroidogenic tissues like the adrenals, and RBCs 



where is the PPP inactive / low activity?

brain, uscle, heart


what is the PPP purpose?

utilizes glucose for: 

1) production of NADPH -> synthesis of cholesterol, fatty acids, steroids, detoxification reactions by cytochrome P450 system 

2) production of ribose  -> produce ntds for RNA and DNA synthesis; NAD, FAD, CoASH 

3) reduction of oxidative stress 


how does NADP / NADPH regulate the PPP?

NADP availability is limiting to the PPP: need it for the 1st 2 steps of the pathway 



how does insulin regulate the PPP?

insulin stimulates the 1st enzyme of the PPP, G6PDH


what is different about the PPP for a tissue that's proliferating vs. not proliferating?

a tissue that is not proliferating - adipose, mammary, RBC - continues the PPP once have made 2 NADPH, 1 CO2, and Ribose 5 P because it can convert that Ribose 5 P into NADPHs 

a tissue that needs both NADPH (for BIOSYNTHESIS) and Ribose-5-P, for RNA and DNA synthesis, can stop the PPP at the Ribose 5 P point 


how does a cell that isn't doing DNA/RNA synthesis proceed on the PPP?

Rearrangement reactions: Convert Ribose 5P back to G6P to NADPH 


3 ribulose 5p -> 2 fructose 6P + 1 Glyceraldehyde 3P 

Fructose 6P -> G6P via isomerase 

Glyceraldehyde 3P = 0.5 G6P, via gluconeogenesis 


THUS, have regenerated 2.5 G6P by rearrangement reactions


what regulates the PPP?

availabilty of G6P and the need for NADPH; NADP availability is crucial for the 2 first steps of the PPP 


why does the RBC have a PPP if it doesn't divide or synthesize much?

for hemoglobin to bind O2 and take to our tissues, the iron of heme must be in the ferrous redox state (Fe2+)

this oxyhemoglobin can be in resonance where an electron from the Fe2+ is given to the O2, making peroxyhemoglobin

this peroxyhemoglobin can dissociate to methemoglobin and free O2, the superoxide anion radical 

this radical can spontaneously form peroxide (hydrogen peroxide at pH=7)

H2O2, H peroxide, is a powerful oxidant that must be rapidly removed to avoid RBC damage 

in order to remove H2O2,  the RBC needs to use catalase and glutathione peroxidase to remove Hydrogen Peroxide from our cells. This requires NADPH, and need a PPP to get that NADPH


what happens to peroxyhemoglobin to form peroxide?

3% of peroxyhemoglobin dissociates to methemoglobin and free superoxide anion radical, which can spontaneously form peroxide (hydrogen peroxide at pH=7) or superoxide dismutases catalyze the conversion of superoxide to H2O2 

H2O2 is a powerful oxidant that must be removed 


what deals w/ H2O2 if it is formed in the RBC?

1) catalase, a heme-containing enzyme, decomposes H2O2 to O2 + H20

2) glutathion peroxidase uses glutathione (GSH) to catalyze reaction, via glutathione peroxidase: 

H2O2+ 2GSH -> 2H20 + GSSG 

this GSSG must be converted back to GSH, via glutahione reducase: 



what is GSH? 


the most important antioxidant in  cells; the most abundant biochemical in cells 

it interacts w/ the reactive oxygen species, i.e. peroxide, via glutathion peroxidase reaction to produce H20 and GSSG, gets rid of the active oxidant 


what is the most common inborn error of metabolism?

what does it cause?

G6PDH deficiency 

high prevalence among African Americans and Mediterranean area ppl

deficiency causes decreased G6PDH activity; severity depends on the mutation, and if individual is exposed to oxidant stress from drugs or foods like fava beans or has severe infection whereby immune system produces oxygen radicals

decreased G6PDH activity -> oxidative damage leading to hemolysis of the RBC and hemolytic anemia 

RBC need G6PDH for both the PPP and glycolysis


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