Carbohydrate Catabolism

Question 1

What is the general formula for carbohydrates?

Answer 1

(CH20)n

Question 2

What are the general features of monosaccharides?

Answer 2

3-9 carbon atoms mostly trisoe, pentose or hexose.
Hydrophilic
Partially oxidised
Aldose= aldehyde group
Ketose= ketone group

Question 3

What are disaccharides?

Answer 3

The condensation of 2 monosaccharides to form a glycosidicidic bond and water
E.g. Lactose, sucrose and maltose

Question 4

Describe the structure of glycogen?

Answer 4

Glucose polymer
Highly branched
a1,4 and a1,6 bonds

Question 5

Describe the structure of starch?

Answer 5

Glucose polymer
From plants
Amylase (a1,4) and amylopectin (a1,4 and a1,6)

Question 6

Describe the structure of cellulose?

Why can’t humans digest it?

Answer 6

Structural polymer in plants
B 1,4 links
Humans do not have enzymes to hydrolyse b1,4 linkages

Question 7

Describe how dietary carbohydrates are digested and absorbed.

Answer 7

Polysaccharide digestion: mouth (salivary amylase) duodenum (pancreatic amylase)
Disaccharides digestion: glycosidase enzymes on brush border of epithelial cells
Monosaccharide absorption: active transport or facilitated diffusion

Question 8

Why must blood glucose be kept at a constant level?

Answer 8

Some tissues have an absolute requirement for glucose
The rate of uptake is dependent on blood concentration
E.g. RBCs, WBCs, lens of eye, kidney medulla absolute
CNS preferential
Some for specialised functions e.g. TAG synthesis in adipose

Question 9

What are the key features of glycolysis?

Answer 9

10 enzyme catalysed steps
Cytoplasm of cells
Generate ATP by substrate level phosphorylation
2 ATP in, 4 out, net gain 2 ATP
Reduces NAD+ to NADH
Produces building block molecules that anabolism
No loss of CO2
Exergonic
Operates anaerobically with one additional enzyme

Question 10

Which three steps of glycolysis are irreversible?

Answer 10

Step one: Glucose –> Glucose-6-phosphate. Requires ATP and hexokinase
Step three: Fructose 6 phosphate –> Fructose 1,6 bisphosphate. Requires ATP and phosphofructokinase
Step 10: Phosphoenol pyruvate –> Pyruvate. Generates ATP. Requires pyruvate kinase.

Question 11

Describe anaerobic respiration?

Answer 11

Inadequate oxygen supply or no mitochondria
Pyruvate reduced lactate by lactate dehydrogenase
LDR regenerates NAD+ without oxygen, allowing glycolysis to proceed

Question 12

How is blood concentration of lactate controlled?

Answer 12

Lactate converted to pyruvate then CO2 or glucose
Lactate produced equals lactate used
If concentration exceeds real threshold buffering capacity is affected causing lactic acidosis

Question 13

What is the cause of lactose intolerance?

Answer 13

Low activity of lactase enzyme

Lactose is not digested

Question 14

Describe galactose metabolism in the liver.

Answer 14

Galactose to galactose 1 phosphate using galactokinase
Galactose 1 phosphate to glucose 1 phosphate using galactose 1PU transferase
Glucose 1 phosphate to glucose-6-phosphate using phosphoglucomutase
Enters glycolysis

Question 15

What causes galactosaemia?

Answer 15

Galactokinase deficiency
Galactose 1PU transferase deficiency
Galactose and galactose 1 phosphate accumulates in tissues (only transferase)

Question 16

What is the consequences of galactose accumulation in tissues?

Answer 16

Reduced to galactitol by aldose reductase
Depletes tissues of NADH
Allows SS bond formation causing cataracts, glaucoma and blindness

Question 17

What is the consequence of galactose 1 phosphate in tissues?

Answer 17

Liver kidney and brain damage

Withholds Pi meaning it is unavailable for ATP synthesis

Question 18

What are the functions of the pentose phosphate pathway?

Answer 18

Produced NADPH (reducing power, maintain free SH groups and used in detoxification)
Produced C5 ribose (Nucleotide synthesis)

Question 19

Describe the pentose phosphate pathway?

Answer 19

Phase 1: Oxidative decarboxylation using glucose 6P dehydrogenase
Phase 2: C5 sugar phosphates –> glycolysis intermediates

Question 20

Describe glucose 6P dehydrogenase deficiency.

Answer 20

Point mutation in X linked gene
Reduced enzyme activity
Low levels of NADPH
Disulphide bridges in red blood cells
Heinz bodies
Premature red blood cell destruction

Question 21

What is the function of pyruvate dehydrogenate?

Answer 21

It converts pyruvate to acetyl~coA so that it can enter the TCA cycle
It is an irreversible reaction which produces CO2

Question 22

How is pyruvate dehydrogenase regulated?

Answer 22

Acetyl~CoA inhibits PDH allosterically
Energy sensitive ATP/NADH inhibit, ADP promotes, allosterically
Insulin activates enzyme by promoting it’s dephosphorylation (Indicates plenty of glucose to be catabolised)

Question 23

What are the functions of the TCA cycle?

Answer 23

Central pathway in sugar, fatty acid, ketone body, alcohol and amino acid metabolism
It is oxidative and it occurs in the mitochondria
Reduces NAD+ and FAD
Produces intermediary metabolites for anabolism

Question 24

How is the TCA cycle regulated?

Answer 24

ADP/ATP ratio, ATP Inhibits
NADH/NAD+ ratio, NADH inhibits
Isocitrate dehydrogenase, NADH inhibits and ADP activates allosterically

Question 25

What is happened by the end of stage III catabolism?

Answer 25

All C-C bonds broken. All C oxidised to CO2
All CH bonds broken. All H transferred to NAD+ and FAD
All energy from bonds stored in ATP/GTP formation and all chemical bond energy of electrons stored in NADH and FAD2H

Question 26

What is oxidative phosphorylation?

Answer 26

High energy electrons are transferred to oxygen by carrier molecules releasing large amounts of free energy
This energy is used to drive ATP synthesis