W6 Other Pathways of Carbohydrate Metabolism

Question 1

what is the glyoxylate cycle

Answer 1

an anabolic variant of the TCA cycle (occur in plants)

Question 2

main difference of glyoxylate cycle from TCA cycle

Answer 2

in TCA cycle, isocitrate undergoes two decarboxylation reactions via isocitrate dehydrogenase and alpha keto glutamarate dehydrogenase > succinate

but for glyoxylate cycle, isocitrate lyase cleaves isocitrate into succinate and glyoxylate > bypass decarboxylation reactions to preserve carbon

Question 3

where does glyoxylate cycle take place

Answer 3

in glyxosome

Question 4

summary of glyoxylate cycle

Answer 4

acetyl coA condenses with oxaloacetate to give citrate > isocitrate in TCA cycle

instead of being decarboxylated, isocitrate cleaved by isocitrate lyase into succinate and glyoxylate

glyoxylate condenses with another acetyl coA to form malate, catalysed by malate synthase > malate oxidised as in in the TCA cycle

succinate passes into mitochondrial matrix and enters TCA cycle to form malate

Question 5

relationship between glyoxylate cycle and TCA cycle

Answer 5

reactions of glyoxylate cycle proceed simultaneously with that of TCA cycle as intermediates pass between these compartments

succinate produced from isocitrate via isocitrate lyase can be transferred to mitochondria > fumarate > malate > oxaloacetate

Question 6

what are the two pathways that malate can take once its produced from succinate

Answer 6

stay in the TCA cycle to produce energy

or pass into cytosol > converted by gluconeogenesis into fructose-6-phosphate (precursor of sucrose)

Question 7

why do plant seeds store fuel as lipids rather than carbohydrates

Answer 7

seeds should be lighter in weight for easier dispersion > lipids is 2-fold lighter than carbohydrates

Question 8

where is NADPH dependent fatty acid synthesis located

Answer 8

in the cytoplasm

Question 9

what is the pentose phosphate pathway

Answer 9

generates NADPH for reactions that require reducing equivalents (electrons) or ribose 5-P for nucleotide biosynthesis

5 carbon sugar intermediates of pathway are reversibly interconverted to intermediates of glycolysis

enzymes of this pathway are particularly abundant in the cytoplasm of liver and adipose cells

Question 10

first step of pentose phosphate pathway

Answer 10

glucose-6-phosphate dehydrogenase oxidises aldehyde of glucose-6P at C1 and reduce NADP+ to NADPH > gluconolacctone

Question 11

step 2 of pentose phosphate pathway

Answer 11

gluconolacctone rapidly hydrolysed to 6-phosphogluconate, a sugar acid with carboxylic acid group at C1, catalysed by 6-phosphogluconolactonase

Question 12

definition of epimerisation and isomerisation

Answer 12

epimerisation: interchange of groups on a single carbon

isomerisation: interchange of groups between carbons

Question 13

step 3 of pentose phosphate pathway

Answer 13

carboxyl group is released as CO2, with electrons transferred to NADP+ > produce NADPH and CO2, forming ribulose-5-phosphate

reaction catalysed by 6-phosphogluconate dehydrogenase

in first 3 steps, total of 2 NADPH formed per mole of glucose-6-phosphate

Question 14

step 4 of pentose phosphate pathway

Answer 14

ribulose-5-phosphate converted into ribose-5-phosphate (R5P) via isomerisation catalysed by ribulose-5-phosphate isomerase

Question 15

step 5 of pentose phosphate pathway

Answer 15

ribulose-5-phosphate epimerase catalyse conversion of ribulose-5-phosphate into xylulose-5-phosphate (Xu5P)

Question 16

enzymes involved in non oxidative portion of pentose phosphate pathway

Answer 16

epimerise, isomerase, transketolase and transaldolase

Question 17

step 6 of pentose phosphate pathway

Answer 17

transketolase first reaction

transketolase catalyse transfer of 2 carbon units from Xu5P to R5P > produce glyceraldehyde-3-phosphate (GAP) and sedoheptulose-7-phoshate (S7P)

reaction requires cofactor TPP

Question 18

step 7 of pentose phosphate pathway

Answer 18

transaldolase transfer 3 carbon unit from S7P to GAP > produce fructose-6-phosphate (F6P) and erythrose-4-phopshate (E4P)

this aldol cleavage occurs between the two OH carbons adjacent to the keto group

important step for regeneration of glycolic intermediates

Question 19

step 8 of pentose phosphate pathway

Answer 19

transketolase second reaction

2 carbon unit from Xu5P transferred to E4P > produced F6P and GAP > both can enter glycolysis or gluconeogenesis

Question 20

mechanism of TPP-dependent transketolase reaction

Answer 20

mechanism involves abstraction of acidic thiazole proton > attack by carbanion at carbonyl carbon of ketone phosphate substrate, expulsion of the GAP product and the transfer of the 2 carbon unit

Question 21

what are the net products of metabolism of 3 mole of 5 RuBP

Answer 21

2 mole of F6P and 1 mole of GAP

Question 22

what is regulation of pentose phosphate pathway controlled by

Answer 22

by cellular concentrations of NADPH, which is a strong inhibitor of G6P dehydrogenase

NADPH oxidised in other pathways > rate of G6P dehydrogenase increase to produce more NADPH

synthesis of liver G6P dehydrogenase is induced by increased insulin:glucagon ratio after carbohydrate meal

Question 23

how is fructose converted to C3 molecules

Answer 23

fructokinase phosphorylates fructose with use of ATP into fructose-1-phosphate > cleaved by aldolase B into dihydroxyacetone-P (DHAP) and glyceraldehyde

dihydroxyacetone-P can enter glycolysis and be converted into GAP > pyruvate + TCA cycle or fatty acid synthesis

glyceraldehyde can be converted into GAP via trios kinase using ATP > sent into glycolysis > pyruvate > TCA cycle or fatty acid synthesis

Question 24

role of pentose phosphate pathway in generation of NADPH

Answer 24

major source of NADPH > provide reducing equivalents for biosynthetic reactions and for oxidation-reduction reactions involved in protection against toxicity of reactive oxygen species (ROS)

Question 25

uses of NADPH

Answer 25

maintain levels of reduced glutathione (GSH) for glutathione-mediated defence against oxidative stress

used for anabolic pathways: fatty acid synthesis, cholesterol synthesis and fatty acid chain elongation

source of reducing equivalents for cytochrome P450 hydroxylation of aromatic compounds, steroids, alcohols and drugs

found in phagocytic cells where NADPH oxidase uses NADPH to form superoxide from molecular oxygen > superoxide generates hydrogen peroxide > kills microorganism take up by phagocytic cells

Question 26

what are the 3 irreversible steps in gluconeogenesis

Answer 26

conversion of pyruvate to phosphoenol pyruvate

conversion of fructose-1,6-biphosphate to F6P

conversion of glucose-6-phosphate to glucose

Question 27

precursors of gluconeogenesis

Answer 27

amino acids (particularly alanine), lactate and glycerol

Question 28

metabolism of gluconeogenic precursors

Answer 28

1. conversion of lactate to pyruvate by lactate dehydrogenase, producing NADH
2. alanine transferase transfers amino group of alanine to alpha ketoglutarate > glutamate > pyridoxal phosphate accepts and donate amino group > forms pyruvate
3. conversion of glycerol to glycerol-3-phosphate by glycerol kinase using ATP > convert to DHAP by glycerol-3-phosphate dehydrogenase using NAD+

Question 29

mechanism of pyruvate carboxylase to convert pyruvate to oxaloacetate

Answer 29

1. ATP-dependent carboxylation of the cofactor to give N-carboxybiotin
2. activated derivative transfers the carboxyl directly to pyruvate > produce oxaloacetate

pyruvate carboxylase uses biotin as cofactor, which is linked to elipson amino group of lysine which swings to transfer the carboxyl

Question 30

why does gluconeogenesis require metabolic transport

Answer 30

pyruvate is converted to oxaloacetate in mitochondria but oxaloacetate cannot be transported across mitochondrial membrane

must first be reduced to malate > transported into cytosol > oxidised back into oxaloacetate before gluconeogensis can continue

known as malate-aspirate shuttle

Question 31

how is pyruvate converted to phosphoenol pyruvate

Answer 31

1. pyruvate converted to oxaloacetate în mitochondria by pyruvate carboxylase, requires atp and CO2
2. oxaloacetate decarboxylated and phosphorylated by phosphoenolpryuvate carboxykinase > produce phosphoenol pyruvate, releasing CO2 and requiring GTP as main energy donor

**the same CO2 used that was fixed in step 1 is released in step 2 > no net fixation of CO2

Question 32

how is fructose-1,6-biphosphate converted to fructose-6-phosphate

Answer 32

via hydrolysis using allosteric enzyme fructose-1,6-biphosphatase

citrate stimulates biphosphatase activity

fructose-2,6-biphosphatase and AMP are inhibitors of the biphosphatase

Question 33

how is glucose-6-phosphate converted back to glucose

Answer 33

via glucose-6-phosphatase

reaction involves a phosphorylated enzyme intermediate, phosphohistidine

glucose-6-phosphatase present in membrane of ER of liver and kidney cells but absence in muscle and brain > gluconeogenesis not carried out in muscle and brain

Question 34

net reaction of gluconeogenesis

Answer 34

2 pyruvate + 2 NADH + 2H+ + 4 ATP + 2 GTP + 6H2O > glucose + 2 NAD+ + 4ADP + 2 GDP + 6Pi

net free energy: -37.7kJ/mol

Question 35

why can gluconeogenesis not be merely the reverse the glycolysis

Answer 35

glycolysis is exogenic of about -74kJ/mol > if gluconeogenesis is merely the reverse, would be strongly endogenic and cannot occur spontaneously

Question 36

what are the two important regulators of gluconeogenesis

Answer 36

acetyl coA and fructose-2,6-biphosphate