Carbohydrates & Glycolysis

Question 1

What are the 4 stages of carb catabolism?

Answer 1

1: breakdown to building block molecules,
2: breakdown to metabolic intermediates – release of reducing power,
3: krebs cycle – release of reducing power,
4: oxidative phosphorylation ATP prod

Question 2

How does stage 1 of carb catabolism occur?

Answer 2

Salivary/pancreas amylase, enzymes on the brush border of the small intestine: lactase, sucrase, isomaltase

Question 3

How does lactose intolerance develop?

Answer 3

PRIMARY: absence of lactase persistence allele – only occurs in adults.

SECONDARY: injury to SI = gastroenteritis, coeliac, crohns, ulcerative colitis.

CONGENITAL: extremely rare, autosomal recessive, cannot digest breast milk

Question 4

How are monosaccharides absorbed?

Answer 4

Active transport by SGLT1 to intestinal epithelial cells, then GLUT2 to blood, then GLUT1-5 from blood into cells

Question 5

Where are GLUT2 found?

Answer 5

Kidney, liver, pancreatic beta cells, SI

Question 6

Where are GLUT4 found?

Answer 6

Adipose tissue, striated muscle (insulin-regulated)

Question 7

Which cells have an absolute glucose requirement?

Answer 7

RBCs (do not have energy stored), neutrophils, kidney medulla, lens of eye (can’t have capillaries blocking site)

Question 8

Why does the brain not have an absolute glucose requirement?

Answer 8

Prefers glucose but can use ketone bodies for some fuel requirements in times of starvation but need time to adapt

Question 9

What does stage 2 of carb catabolism produce and how?

Answer 9

Pyruvate by glycolysis

Question 10

Glycolysis can be broken down into what 2 phases?

Answer 10

Phase 1 = investment phase, 2 ATP in, making glucose –ve increasing reactivity to permit subsequent steps, large –ΔG so irreversible, committing step.

Phase 2 = payback phase, 4 ATP out, substrate level phosphorylation, large –ΔG so irreversible.

Question 11

How many enzymes are used in glycolysis?

Answer 11

10

Question 12

In what stage of glycolysis is NADH produced?

Answer 12

Phase 2 = payback phase

Question 13

What is the function of hexokinase?

Answer 13

phosphorylates hexoses (6C sugars), 
forming G-6-P

Question 14

What is the function of phosphofructokinase-1?

Answer 14

regulatory allosteric enzyme made of 4 subunits,
phase 1 = investment phase,
involved in committing step,
conversion of fructose 6-phosphate and ATP to fructose 1,6-bisphosphate and ADP,

inhib by high ATP and glucagon,
stim by high AMP and insulin

Question 15

Outline the function of pyruvate kinase

Answer 15

catalyses the final step of glycolysis:
stim by high insulin,

transfer of a phosphate group from phosphoenolpyruvate (PEP) to adenosine diphosphate (ADP) = pyruvate and one molecule of ATP

Question 16

During glycolysis how many irreversible steps are there?

Answer 16

3

Question 17

In what pathology is the rate of glycolysis greater?

Answer 17

Cancer – this can be measured by uptake of radioactive modified hexokinase substrate

Question 18

Which reaction in glycolysis is NAD+ required?

Answer 18

Reaction 6

Question 19

How is NAD+ regenerated?

Answer 19

Lactate dehydrogenase

Question 20

What does NAD+ and NADH do?

Answer 20

Nicotinamide adenine dinucleotide exists in two forms, an oxidized and reduced form abbreviated as NAD+ and NADH respectively.

In metabolism, NAD+ is involved in redox reactions, carrying electrons from one reaction to another

Question 21

What reaction does lactate dehydrogenase catalyse

Answer 21

NADH + H+ + pyruvate NAD+ + lactate

Question 22

What is hyperlactaemia?

Answer 22

Lactate 2-5mM, below renal threshold, no change in blood pH as buffering capacity

Question 23

What is lactate acidosis?

Answer 23

> 5mM, above renal threshold, blood pH lowered

Question 24

Deficiencies in which 3 enzymes cause galactosaemia?

Answer 24

Galactokinase,

UDP-galactose epimerase,

uridyl transferase

Question 25

When fructokinase is absence what occurs?

Answer 25

Essential fructosuria: fructose present in the urine but no clinical signs

Question 26

Explain fructose intolerance

Answer 26

Aldolase is missing, fructose accumulates in the liver = damage

Treat = remove fructose from diet

Question 27

The pentose phosphate pathway is an important source of what?

Answer 27

NADPH

Question 28

What does the pentose phosphate pathway result in?

Answer 28

C5-sugar ribose required for the synthesis of: nucleotides, DNA, RNA

Question 29

The rate limiting enzyme of the pentose phosphate pathway is?

Answer 29

Glucose 6-phosphate dehydrogenase

Question 30

What part of carb catabolism can operate anaerobically?

Answer 30

Glycolysis, with the addition of the enzyme LDH

Question 31

What are the main features of glycolysis?

Answer 31

6C sugar to 2x 3C sugar = pyruvate,

ATP needed to be invested to activate glycolysis,

lower down the pathway is an oxidation step in which NAD taking reducing equivalents from intermediates,

lower down there is steps of substrate level phosphorylation = phosphate removed transferring to ADP making ATP,

galactose and fructose entering the pathway – being metabolised,

if energy is high glycolysis is switched down = substrate is diverted out – prod NADPH,

if no O2 to re-oxidase NADH can use LDH

Question 32

When ATP is high in skeletal muscle, energy may be temp stored in which substrate?

Answer 32

Phosphocreatine = stores phosphate bond energy

Question 33

What is the chem formula for glucose?

Answer 33

C6 H12 O6

Question 34

Which glucose transporter is regulated by insulin?

Answer 34

GLUT4

Question 35

What is the net prod of ATP from 1 molecule of glucose in glycolysis?

Answer 35

2