Biochem exam: glycolysis/pyruvate catabolism

monosaccharide metabolism

All monosaccharides have to undergo

phosphorylation prior to being metabolized

by specific pathway

– Phosphorylation reactions are driven by ATP

and specific kinases

• Once phosphorylated, monosaccharides can

then enter various metabolic pathways

depending upon tissues; time of day;

fasting, feasting, physical activity; and/or

hormones

glycolysis

Pathway for the catabolism of glucose into energy

predominantly

• Occurs in the cytosol of all cells

• Overall: glucose → pyruvate, NAD+ → NADH +

H+ and ADP → ATP

– Pathway is also involved in the conversion of

monosaccharides → glycerol and fatty acids

(triacylglycerol (fat) synthesis) when ATP

concentrations are high

– Works under both aerobic and anaerobic

conditions (doesn’t need O2)

• In most tissues, other than liver and RBCs,

glycolysis is turned on by insulin following

a meal or by glucagon and epinephrine

during fasting and physical activity

• Thus it is turned on in differing cells/tissues

all the time

glycolysis: initiating step

Phosphorylation

– Glucose + ATP → glucose 6-phosphate + ADP

• Hexokinase – all cells

• Glucokinase – inducible, only in liver following a

meal

• “Rate limiting” (oneway) and regulating enzymes for

glucose metabolism

• Glucose 6-phosphate is thus the starting point for

several metabolic pathways including; glycolysis,

glycogenesis, pentose phosphate pathway

step 1

Glucose 6-phosphate  fructose 6-

phosphate

– Phosphohexose isomerase

Glycolysis step 2

Fructose 6-phosphate + ATP →

fructose- 1,6-bisphosphate + ADP

– Phosphofructokinase – “rate

limiting/regulating enzyme”

Glycolysis step 3a/3b

Fructose 1,6-bisphosphate 

glyceraldehyde-3-phosphate +

dihydroxyacetone phosphate (DHAP)

– Aldolase A

• DHAP  glyceraldehyde 3-phosphate

– Triosephosphate isomerase

– In general, 2 molecules of glyceraldehyde 3-

phosphate are produced from one glucose

molecule during glycolysis when glycerol is not

needed

glycolysis step 4

Glyceraldehyde 3-phosphate + NAD+ + Pi 

1,3-bisphosphoglycerate (BPG) + NADH + H+

– Glyceraldehyde 3-phosphate dehydrogenase

– Pi = inorganic phosphate

– This and the remaining steps occur twice for one

glucose

– Fate of NADH + H+ depends upon O2 availability

gly

glycolysis step 5

1,3-BPG + ADP  3-phosphoglycerate +

ATP

- Phosphoglycerate kinase

- Substrate level phosphorylation

- Make 2 ATPs since reaction occurs twice

glycolysis step 6

3-phosphoglycerate ↔ 2-phosphoglycerate

– Phosphoglycerate mutase

glycolysis step 7

2-phosphoglycerate ←>

phosphoenolpyruvate

(PEP) + H2O

– Enolase

glycolysis step 8

P2-phosphoglycerate 

phosphoenolpyruvate

(PEP) + H2O

– Enolasegl

glycolysis regulation

Hexokinase (glucokinase)

• Phosphofructokinase

• Pyruvate kinase

– All irreversible steps

– In skeletal muscle: pathway is inhibited by high

ATP and NADH concentrations (products)

(when you don’t need energy; resting state)

pyruvate catabolism: under anaerobic conditions

Under strictly anaerobic conditions

• Pyruvate + NADH + H+  lactate +

NAD+

– Lactate dehydrogenase (LDH)

– NADH + H+ for this reaction was produced in

step 4

– RBC’s are constantly producing low levels of

lactate and releasing it into the blood stream

in order for them to continue glycolysis

– Other working cells only do this reaction

when there is an O2 deficiency

pyruvate catabolism: under aerobic conditions

Under strictly aerobic conditions

• Pyruvate is transported through the

mitochondrial membranes into the matrix

• Pyruvate dehydrogenase complex – multi-

enzyme complex

Pyruvate Dehydrogenase Complex: step 1/2

Step 1: pyruvate + thiamin diphosphate (TDP)

→ hydroxyethyl TDP + CO2

– Pyruvate dehydrogenase

– TDP = coenzyme form of thiamin (vitamin B1)

• Step 2: hydroxyethyl TDP + oxidized

lipoamide → acetyl lipoamide + TDP

– Dihydrolipoyl transacetylase

– Oxidized lipoamide = coenzyme form of alpha

lipoic acid (a B vitamin)

Pyruvate Dehydrogenase Complex: step 3/4/5

Step 3: acetyl lipoamide + coenzyme A (CoA)

→ acetyl CoA + dihydrolipoamide

– CoA = coenzyme form of pantothenic acid

(vitamin B5)

• Step 4: dihydrolipoamide + FAD → oxidized

lipoamide + FADH2

– Dihydrolipoyl dehydrogenase

• Step 5: FADH2 + NAD+ ←> FAD + NADH +

H+