Dong Hoc Xu Ly Sinh Hoc

8/10/2019 Dong Hoc Xu Ly Sinh Hoc

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Engineering of BiologicalProcesses

Lecture 3: Yields and stoichiometry

Mark Riley, Associate Professor

Department of Ag and BiosystemsEngineering

The University of Arizona, Tucson, AZ

2007


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Objectives: Lecture 3

Biosynthetic processes (anabolic)

Case studies - cholesterol

Stoichiometry and modeling cellular

requirements

"You are what you eat"


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Stoichiometry

Provides information on fundamental

constraints

Substrate conversion to product

Cell mass from substrate


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Yields and yield coefficients

Mass based = kg of this from kg of that

Y (output / input)

Y x/s

Y p/s

Y ATP/O2

Ymx/smaximal yield of cell mass from

substrate


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Yield

Overall

Instantaneous

Ratio of rates

Ratio of yields

Theoretical = Y

Observed = Y

YIELD


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Cell metabolism

Y lactate / glucose = ranges from 2 to 0 based on environment

The basic reaction is:

Glucose + 2 Pi+ 2 ADP 2 Lactate + 2 ATP + 2 H2O


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Glucose [g/L]

Bacterial

dry cell weight

[mg/L]Slope = dX/dS

7 (mg/L) / (g/L)

Yield of cell mass from substrateY x/s


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Glucose [mM]

Bacterial

dry cell weight

[g/L]

Aerobic

Yx/s=58 mg/mol

Anaerobic

Yx/s=22 mg/mol


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Cell compositionDry weight vs. wet weight

70% of the composition is water

Dry weight consists of:

Element E. co l i Yeast

CONH

PSK

NaOthers

50%20%14%8%

3%1%1%1%


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In a very simplistic interpretation of

metabolism, the following applies:

Cells + medium + O2(sometimes) more cells +

product + CO2+ H2O

Medium contains sugars, amino acids,

cofactors and the elements in the previous

table.


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Stoichiometric calculations

Based on 1 mole of C in the input

CHm

On+ a O

2+ b NH

3

c CHaObNd+ dH2O + eCO2

This is normalized to 1 mole of C.

Could also be normalized to 1 mole of the C source compound

Perform elemental balances to determine theunknown values of the cofactors


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Example

C6H12O6+ a O2+ b NH3

c C4.4H7.3O1.2N0.86+ dH2O + eCO2

2/3 of the glucose C goes to biomass

What are the stoichiometric coefficients,

and Yx/s, Yx/O2? MWglucose = 180MWcell = 89.62

MWoxygen = 32MWammonia = 17


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Generalized growth reaction C6H12O6+ a NH3+ b O2aCH1.8O0.5N0.2+

bCHxOyNz+ gCO2+ dH2O

Normalized to 1 mole of carbon source compound Where a, b, a, b, g, d, x, y, z depend on the type of cellinvolved.

a, b, a, b, g, d, are stoichiometric coefficients

When little info is available about cell composition, usean approximated cell composition of

CH1.8O0.5N0.2

This yields a MW of a cell ~ 24.6


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Generalized growth reaction

180

6.24Y

sx

a

C6H12O6+ a NH3+ b O2 aCH1.8O0.5N0.2+

bCHxOyNz+ gCO2+ dH2O

g of cells from g of glucose


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Lack of information

Unfortunately, the elemental balances

often do not provide enough information

to completely solve for the stoichiometriccoefficients.


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Respiratory quotient

RQ = YCO2/O2

Molar basis Moles of CO2produced from moles of O2

Provides information on the metabolic state of the cell

A high RQ means that much CO2is produced andhence the metabolism is operating at high efficiency


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Aerobic metabolism

CHmOn+ a O2+ b NH3

c CHaObNd+ d CHxOyNz+ eH2O + fCO2

RQ = ?


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Degree of reduction

Electron balance

g = # of available electrons / g of atomic C

g Or, this can be described as:

g = # of available electrons / # of Cs

Provides another independent equation


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Degree of reduction

C = 4

H = 1

N = -3 O = -2

P = 5

S = 6

CO2= +4 (C) + -2 (O) = 0

C6H12O6= 6(4) + 12(1) + 6(-2) = 24 g= 24 / 6 (# carbon atoms) = 4

C2H5OH = 2(4) + 6(1) + (-2) = 12

g= 12 / 2 (# carbon atoms) = 6


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Exampleyeast grown on glucose

C6H12O6+ 0.48 NH3+ 3 O2

0.48CH1.8O0.5N0.2+ 3.12CO2+ 4.32H2O

To grow yeast to 50 g/L in a 100,000 L reactor, determine:

a) mass of glucose and ammonia required

b) O2required

c) Yx/sand YX/O2

MWglucose = 180MWcell = 24.6

MWoxygen = 32

MWammonia = 17


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HW #1 questions

1) What kind of cell would you use to produceandrostenedione? Your answer shoulddescribe the attributes of such a cell (don't

just state, "a cell that produces andro"). An

answer longer than 4 sentences is too much.2) Producing cholesterol is an energy intensiveprocess. How much energy (in terms of # of

ATP molecules) is consumed in producingone cholesterol molecule from a source of

glucose?

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