Carbohydrate Metablism II

Question 1

what kind of bond is acetyl-CoA

Answer 1

• high energy thioester bond

- can be used to drive other reactions when hydrolysis occurs

Question 2

how is acetyl-CoA formed?

Answer 2

from pyruvate via pyruvate dehydrogenase complex

Question 3

explain the pyruvate dehydrogenase complex

Answer 3

five step complex in the mitochondrial matrix that forms (and is inhibited by) acetyl-CoA and NADH

1. pyruvate dehydrogenase
2. dehydrolipoyl transacetylase
3. dehydrolipoyl dehydrogenase
4. pyruvate dehydrogenase kinase
5. pyruvate dehydrogenase phosphatase

Question 4

pyruvate dehydrogenase (PDH)

Answer 4

oxidizes pyruvate, creating CO2

-requires thiamine pyrophosphate (vitamin B1, TPP) and Mg2+

Question 5

dehydrolipoyl transacetylase

Answer 5

oxidizes the remaining two-carbon molecule using lipoic acid and transfers the resulting acetyl group to CoA, forming acetyl-CoA

Question 6

dehydrolipoyl dehydrogenase

Answer 6

uses FAD to reoxidize lipoic acid, forming FADH2

-FADH2 can later transfer electrons to NAD+, forming NADH that can go into the ETC

Question 7

pyruvate dehydrogenase kinase

Answer 7

phosphorylases PDH when ATP or acetyl-CoA levels are high, turning it off

Question 8

pyruvate dehydrogenase phosphatase

Answer 8

dephosphorylates PDH when ADP levels are high, turning it on

Question 9

what can acetyl-CoA be formed from?

Answer 9

fatty acids that enter the mitochondria using carriers

• carbon skeletons of ketogenic amino acids
• ketone bodies
• alcohol

Question 10

explain the process of going from fatty acids into acetyl-CoA

Answer 10

• fatty acid couples with CoA in the cytosol to form fatty acyl-CoA, which moves to the intermembrane space
• acyl(fatty acid) group is transferred to carnitine to form acyl-carnitine, which crosses the inner membrane
• acyl group is transferred to a mitochondrial CoA to re-form fatty acyl-CoA, which can undergo beta oxidation to form acetyl-CoA

Question 11

where does the citric acid cycle take place?

Answer 11

mitochondrial matrix

Question 12

what is the main purpose of the citric acid cycle?

Answer 12

to oxidize carbons in intermediates to Co2 and generate high energy electron carriers (NADH and FADH2) and GTP

Question 13

citrate synthase

Answer 13

couples acetyl-CoA to oxaloacetate and hydrolyzes the product forming citrate and CoA-SH
-regulated by negative feedback from ATP, NADH, succinyl-CoA, citrate

Question 14

what does aconitase isomerize?

Answer 14

isomerizes citrate to isocitrate

Question 15

isocitrate dehydrogenase

Answer 15

oxidizes and decarboxylates isocitrate to form alpha-ketoglutarate
-generates the first CO2 and first NADH of the cycle
RATE LIMITING STEP OF CAC
-inhbitors: atp and NADH
-activators: ADP and NAD+

Question 16

alpha-ketoglutarate dehydrogenase complex

Answer 16

metabolizing alpha-ketoglutarate to form succinyl-CoA

• generates the second CO2 and NADH of the cycle
• inhibited: NADH and succinyl Co-A
• activated: ADP and Ca2+

Question 17

succinyl-CoA synthetase

Answer 17

hydrolyzes the thioester bond in succinyl-CoA to form succinate and CoA-SH
-generates the one GTP generated in the cycle

Question 18

succinate dehydrogenase

Answer 18

oxidizes to form fumarate

Question 19

what is fumarate?

Answer 19

it is a flavoprotein that is anchored to the inner mitochondrial membrane because it requires FAD, which is then reduced to form the one FADH2 that is generated in the cycle

Question 20

fumarase

Answer 20

hydrolyzes the alkene bond of fumarate, forming malate

Question 21

malate dehydrogenase

Answer 21

oxidizes malate to oxaloacetate

-generates the third and final NADH of the cycle