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Steering carbon toward your product

Science · Microbiology · ages 23-24

Name ______________________ Date ____________

A native branch and a target branch both use intermediate I. Which process is a competing drain of I away from the target branch?
- I is converted into a native byproduct
- I is supplied more slowly from upstream
- I is converted into the desired target product
A carbon-only model converts one 6-carbon substrate into one 4-carbon product. Only CO2 carries the remaining carbon out, and no carbon accumulates. Which CO2 output completes the balance?
- One CO2 molecule per substrate molecule
- Three CO2 molecules per substrate molecule
- Two CO2 molecules per substrate molecule
Inducing a production pathway after biomass growth, by itself, makes product formation necessary for growth.
Circle one: True False
A host produces 30 mmol ATP/hour. Growth and maintenance use 18 mmol ATP/hour. A model pathway uses 3 mol ATP per mole of product. With no stored ATP available, what upper bound does this ATP budget set?
- 10 mmol product/hour
- 12 mmol product/hour
- 4 mmol product/hour
A branch competing for your target intermediate also supplies an essential growth precursor. Which change is a reasonable first test for redirecting flux?
- Delete the branch and remove its growth precursor supply
- Reduce branch activity while checking growth and flux
- Keep branch activity fixed and assume target flux must rise
A model pathway uses 3 mol NADPH per mole of product. Production has 15 mmol NADPH/hour available after other cellular needs, with no stored reserve. What maximum product flux does this NADPH budget allow?
- 5 mmol product/hour
- 15 mmol product/hour
- 45 mmol product/hour
In a model, growth is impossible at zero product flux. What does this growth coupling establish?
- Every faster-growing cell has the highest product yield
- Growth requires some positive product flux
- Product is made only after growth has completely stopped
After many generations, mutants lacking a costly production pathway become common. They grow faster than producers in this culture. Which process explains their increasing frequency?
- Pathway loss guarantees a higher product yield per cell
- Slower growth makes the producing cells divide more often
- Selection favors the faster-growing mutants

Answer key

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Steering carbon toward your product W1-mt_fjz3EA-dDa-s1

I is converted into a native byproduct · The native branch consumes the same intermediate needed by the target branch. Slower upstream supply is a different constraint.
Two CO2 molecules per substrate molecule · Six carbons enter and four leave in product. The remaining two carbons leave in two CO2 molecules, with one carbon in each.
False · Induction controls when the pathway is expressed. Making growth require product formation is a separate design condition.
4 mmol product/hour · Production has 30 - 18 = 12 mmol ATP/hour available. Dividing by the cost of 3 gives an upper bound of 4 mmol product/hour.
Reduce branch activity while checking growth and flux · A partial reduction can test whether less competing consumption helps production while retaining enough precursor supply for growth. A gain is not guaranteed.
5 mmol product/hour · Each mole of product needs three moles of NADPH. Divide the available NADPH rate by three: 15 / 3 = 5 mmol product/hour.
Growth requires some positive product flux · The model excludes growth without product formation. It does not specify how product yield ranks across cells or strains.
Selection favors the faster-growing mutants · The mutants have a stated growth advantage in this culture. Selection can increase their share of the population while production falls.