ch. 14 pentose phosphate pathway

what is the pentose phosphate pathway (other names?)

phosphogluconate pathway, hexose monophosphate pathway

oxidized glucose 6-phosphate, producing pentose phosphates and NADPH

what is NADPH needed for

needed for fatty acid synthesis

what is an important product of the pentose phosphate pathway and its uses

ribose 5-phosphate

used in nucleotides, coenzymes, DNA, RNA

what kinds of cells use the PPP and what do they need from it?

rapidly dividing cells - RNA, DNA, coenzymes

cells that do fatty acids synthesis (liver, adipose, mammary gland) - need NADPH

cells that synthesize cholesterol and steroid hormones (liver, adrenal glands, gonads) - need NADPH

what does glucose 6-phosphate dehydrogenase (G6PD) do

catalyze oxidation of G6P to 6-phosphoglucono-(delta)-lactone

uses NADP+ as electron acceptor, generating NADPH

needs Mg2+

add oxygen, take away hydrogen …?

what does lactonase do

catalyze hydrolysis of lactone to free acid 6-phosphogluconate

what does 6-phosphogluconate dehydrogenase do

catalyze oxidation and decarboxylation of 6-phosphogluconate to form ribulose 5-phosphate and NADPH

what does phosphopentose isomerase do

convert ribulose 5-phosphate to its aldose isomer, ribose 5-phosphate

what’s the overall equation for the PPP

glucose 6-phosphate + 2 NADP+ + 2H2O → ribulose 5-phosphate + CO2 + 2 NADPH + 2H+

what does ribulose 5-phosphate epimerase do

epimerizes ribulose 5-phosphate to xylulose 5-phosphate

the conversion of _____ ______ to ________ begins the oxidative cycle again

pentose phosphates

glucose 6-phosphate

what reactions convert PP to G6P

transketolase reaction 1, transaldolase reaction, transketolase reaction 2, etc (from glyceraldehyde 3-phosphate to G6P)

describe the first transketolase rxn

transketolase enzyme catalyzes transfer of a 2C fragment from ketose donor to aldose acceptor

yields sedoheptulose 7-phosphate

TPP is coenzyme

describe transaldolase rxn

transaldolase enzyme catalyzes condensation of 3C fragment from sedoheptulose 7-P and glyceraldehyde 3-P

yields fructose 6-phosphate and (tetrose) erythrose 4-phosphate

describe second transketolase rxn

transketolase enzyme catalyzes reaction between xylulose 5-phosphate and erythrose 4-phosphate

yields fructose 6-P and glyceraldehyde 3-P

TPP coenzyme

what is the role of thiamine pyrophosphate (TPP) in transketolase rxn

stabilizes 2C carbanion in transketolase reaction

what stabilizes in the transaldolase rxn

protonated Schiff base stabilizes the carbanion intermediate in transaldolase rxn

which steps of the oxidative PPP are irreversible

first and third steps

(nonoxidative reactions are readily reversible)

what is the reductive pentose phosphate pathway

converts hexose phosphates to pentose phosphates

photosynthetic assimilation of CO2 by plants

essentially the reversal of nonoxidative reactions of oxidative PPP

what determines whether glucose 6-phosphate enters either glycolysis or the PPP?

RELATIVE CONCENTRATIONS OF NADP+ AND NADPH

if NADPH is high, PPP inhibited, it will do glycolysis

if not, it will do PPP and not glycolysis

what are the problems caused by a thiamine deficiency

thiamine is precursor to TPP

beriberi = swelling, pain, paralysis, death

Wernicke-Korsakoff syndrome = problems with voluntary movements. common among alcoholics due to decreased intestinal absorption of thiamine