Glucose Metabolism

macronutrients vs micronutrients

• macronutrients is needed in large amounts (e.g. carbs, proteins, fats)

• micronutrients is needed in smaller amounts (e.g. minerals, vitamins)

what and where is glucose stored

glycogen in the liver and muscles

what and where is fat stored

triacylglycerols in adipose cells for long term energy storage

what are where are amino acids stored

NO storage

excess amino acids are broken down to urea for excretion and oxidised for energy

true or false: glucose is NOT a precursor of metabolic intermediates for reactions

false - it is a precursor (a susbtance form which another is formed)

what happens in glycolysis

• glucose undergoes series of oxidation-reduction reactions to form 2×3C pyruvate molecules

glycolysis is the breakdown of glucose and occurs in 10 steps. what are the 2 phases

• preparatory phase

• payoff phase

what happens in the preparatory phase

1) glucose → glucose-6-phosphate (G6P)

• uses ATP

• glucose is phosphorylated at OH groups on C6

2) G6P → fructose-6-phosphate (F6P)

• conversion

3) F6P → fructose-1,6-bisphospate (F16BP)

• uses ATP

• phosphorylation at C1 of F6P

• for both these phosphorylations (in step 1 and 3), ATP is the phosphoryl group donor

4) F16BP is split into 2 different 3C molecules:

• glyceraldehyde 3-phosphate (GP3)

• dihydroxyacetone phosphate → glyceraldhyde 3-phosphate

overview of preparatory phase

• glucose converted into 2 × 3C molecules

• 2 molecules of ATP used

  • this raises free-energy content of intermediates

  • carbon chains are converted into glyceraldehyde 3-phosphate

what happens in payoff phase

6) 2 x GP3 -→ 2 × 1,3-biophosphoglycerate

• GP3 molecules are oxidised and phosphorylated by inorganic phosphate (Pi) and NAD+ is used)

7-10) 2 × 1,3-biophosphoglycerate → 2 x pyruvate

• energy is released as this conversion takes place

• 2ADP + 2Pi → 2ATP

overview of payoff phase

• 4ADP → 4ATP

• NAD⁺ → 2NADH + 2H⁺

write the overall reaction of glycolysis

Glucose + 2NAD⁺ + 2ADP + 2Pi → 2 pyruvate + 2NADH + 2H⁺ + 2ATP + 2H2O

what is the net yield of ATP from glycolyisis

• preparatory phases uses 2ATP

• payoff phase releases 4ATP

• net overall yield is 2ATP per glucose molecule

• energy is conserved in formation of 2NADH electron carriers per glucose

true or false: glycolysis only releases a small fraction of the total energy available in glucose molecule

true - pyruvate still contains most of the chemical potential energy

what is a common dietary carb

starch - this is hydrolysed into monsaccharides to be taken up from intestine

in aerobic conditions, what happens to the pyruvate

oxidised to acetate (acetyl CoA) which enters citric acid cycle

in anaerobic conditions, what happens to pyruvate

1) lactic acid fermentation

2) ethanol fermentation

what happens in lactic acid fermentation

• pyruvate is reduced to lactate

• regenerates NAD⁺ from NADH used for glycolysis

where is lactate formed and then transported

formed by erthrocytes (skeletal muscles)

carried in the blood to liver where its converted into glucose

what happens why large amounts of lactate is produced during vigorous muscular contraction

acidification of blood due to lactic acid build up

what is ethanol fermentation

• pyruvate → ethanol +CO2 by yeast

what enzyme metabolises ethanol in the liver

alcohol dehydrogenase

• catalyses oxidation of ethanol with reduction of NAD+ to NADH

glucose is the main fuel source for which systems and organs

brain, nervous system etc so high demands of glucose

supply of glucose is not always sufficient. what happens to glucose

glucose synthesised from non-carb precursors (e.g. lactate, pyrivate, glyercol) which is achieved by gluconeogenesis

what is gluconeogenesis

converts pyruvate to glucose

where does gluconeogenesis occur

in the liver mainly and in renal cortex, epithelial cells lining the intestine too

what are the reactions of gluconeogenesis in comparisons to glycolysis

• 7/10 reactions are the the reverse of glycolysis

which reactions of glycolysis are irreversible (3/10) and therefore cannot be used in glucogeogenesis

1) conversion of glucose to glucose 6-phosphate by hexokinase

2) phsophorylation of fructose 6-phosphate to fructose 1,6-bisphosphate by phosphofructokinase-1

3) conversion of phosphenolpyruvate to pyruvate by pyruvate kinase

what happens if these reactions cannot happen in gluconeogenesis

• they are bypassed by a separate set of enzymes, catalysing reactions that are irreversible in glucose synthesis

what is gluconeogensis activated by:

break down of fats also yield this product in the mitochondria - this means oxidation of glucose is not needed

acetyl-CoA

what happens when the body does not need ATP

glucose 6-phosphate enters pentose phosphate pathway

what is the pentose phosphate pathway

• alternative pathway for glucose oxidation

• yields pentoses for nuclotide synthesis and reduced cofactors for biosynthesis of fatty acids

what does pentose phosphate pathway produce

• 5C sugar ribose phosphate makes RNA, DNA and coenzymes

• NADPH creates fatty acids, cholesterol and steroid hormones

where does pentose phosphate pathway take place

cytosol

true or false: G6P produces NADPH via hexose phosphate pathway using G6P dehydrogenase

true

what is glycogen

polymeric compact storage of glucose when glucose is in excess and easily made available when needed

in what form is glycogen stored as

beta-granules

true or false (about the breakdown of glycogen): glucose units on outer branches of glycogen are broken down by enzymes such as glycogen phosphorylase

true - glucose 6-phosphate is produced

what happens to glucose 6-phosphate produced in skeletal muscles

enters glycolysis and acts as energy storage for muscular contractions

where does glycogen synthesis occur

in liver and skeletal muscles

glucose + ATP → G6P + ADP. what is the G6P converted into

G1P which attaches to glycogen branch

what 2 roles does glycogenin protein have

• primer - new chains are assembled

• enzyme 0 catalyses their assembly

true or false: glycogen particles begin with autocatalytic formation of glycosidic bond between glucose and Tyr residue of protein glycogenin

true - addition of more glucose residues form primer which is acted on by glycogen synthase

how is glycogen phosphorylase regulated

by hormone-stimulated phosphorylation

glycogen phosphorylase has 2 interconvertible forms

1) glycogen phosphorylase a

2) glycogen phosphorylase b

which one is more active

glycogen phosphorylase a

during muscular activity in resting muscles, what happens to phosphorylase b

epinephrine triggers phosphorylation of glycogen phosphorylase b converting it into glycogen phosphorylase a. this stimulates glycogen breakdown and glycolysis which provides ATP

in the liver, what does glucagon trigger

triggers phosphorylation of glycogen phosphorylase b converting it into glycogen phosphorylase a

true or false: glucagon stimulates glycogen breakdown and gluconeogenesis while blocking glycolysis

true