L17 - Synthesis and Mobilisation of Carbohydrate Stores

Question 1

What is starch?

What is its function and why is it useful for humans?

Answer 1

• Insoluble carbon store built through polymerisation of glucose units
• Dominant carbohydrate store in plants. Needed for when photosynthesis is not possible
• Carbohydrate stores are essential food staples e.g. maize, potato, rice, wheat

Question 2

How is the rate of starch breakdown altered?

Answer 2

• Starch breakdown is linear
• Rate of breakdown decreases as night length increases
• Starch reserves used at constant rate for growth throughout the night

Question 3

Describe the two types of polymer within the starch granule

Answer 3

Amylose:
- Glucose monomers linked via 𝛂-1,4 glycosidic bonds
- Lightly branched, branches at 𝛂-1,6 bonds
- Roughly 1000 glucose units per molecule
- E.g. biodegradable packing, thickener, fried snacks

Amylopectin:
- Chains of glucose monomers linked via 𝛂-1,4 bonds
- Highly branched with 𝛂-1,6 bonds joining chains
- Average brach length = 20-25 glucose monomers
- Total molecule has 10^4 - 10^5 glucose molecules
- E.g. adhesive, paper strength, improved freeze thaw

Question 4

Further describe the structure of starch

Answer 4

• Grains made up of alternating crystalline and amorphous lamellae
• Crystalline sections from aligned double helices of amylopectin

Question 5

Give the two categories of starch

Answer 5

Transitory starch
- generated in photosynthetic cells inside chloroplast
- rapid production
- nearly all broken down at night for respiration + growth

Storage starch
- Made in non-photosynthetic cells

Question 6

Give the two possible enzymatic routes for starch synthesis

Answer 6

1) Glucose 1-P + ATP to ADPglucose + PPi (ADPG pyrophosphorylase - ADPG PPase)

ADPglucose + n(𝛂 1,4 glucan) to starch (Starch synthase)

2) Glucose 1-P + n(𝛂 1,4 glucan) to Starch + Pi (Starch phosphorylase)

Question 7

Which of the two possible enzymatic routes to starch is used for starch synthesis in plants?

Outline the evidence

Answer 7

• All starch made by ADPG PPase and Starch Synthase
• Thermodynamics and comparative biology indicate Starch Phosphorylases degrades starch
• Arabidopsis mutant lacked starch due to single mutation in gene encoding ADPG pyrophosphorylase

Question 8

Name the enzyme that catalyses the formation of 𝛂 1,6 bonds

Describe a famous experiment that identified this enzyme

Answer 8

• Branching enzyme
• Wrinkled pea mutant first identified by Mendel contains transposon insertion in isozyme of Branching Enzyme
• Mutation causes low Branching Enzyme activity and low starch content
• Peas collapse upon drying and have higher relative amylose content

Question 9

How is transitory starch synthesis activated?

Describe the mechanism

Draw the relevant diagram

Answer 9

• Activated by photosynthesis
• Fructose 1,6-bisPhosphatase (FbPase) enzyme in chloroplast allows hexose phosphates required for starch synthesis
• FbPase activated/deactivated by following sequence:

1) In light, PSI reduces ferredoxin
2) Redox cascade occurs from ferredoxin to thioredoxin to FbPase
3) Disulphide bridge within FbPase is reduced, activating FbPase

See diagram on pg 12

Question 10

What other factors, apart from light, can influence the rate of starch synthesis.

Provide evidence

Answer 10

• Starch synthesis upregulated when sucrose production inhibited, maximising photosynthesis

1) Sucrose synthesis declines = less Pi returned to chloroplast by TPT
2) ADPG PPase allosterically activated by increase in 3PGA:Pi ratio

• Transgenic tobacco and potato with antisensed TPT showed increased starch
• Overexpressed TPT decreased starch

Question 11

Explain how storage starch is synthesised in 5 steps

Draw the relevant diagram

Answer 11

1) Sucrose converted to UDPglucose + fructose by Sucrose Synthase (in some sink tissue Invertase used to hydrolyse sucrose e.g. Tomato)

2) Fructose converted to Fructose 6-Phosphate by Hexokinase

3) UDPglucose converted to hexose phosphates (G1P initially) by UDP-Glucose pyrophosphorylase (UDPG-PPiase), high levels in cytosol of storage tissues

4) Hexose-P translocator with strict counter exchange for Pi imports hexose-Ps to amyloplast

5) Hexose Ps form starch via same mechanism as in transitory starch (ADPG PPiase, starch synthase, branching enzyme abundant in amyloplasts)

See diagram on pg 13

Question 12

Some plants don’t store large amounts of insoluble starch, what do they do instead?

Answer 12

• Store fructans in vacuole instead
• Fructans: water soluble, non reducing polymers of fructose
• Synthesis similarly induced by high sucrose
• Implicated in cold and drought tolerance
• Forage grasses store fructans in leaves, not easily digestible, impacting animal husbandry