P/O-quotient

Where biosynthesis of a product requires the net input of energy, the theoretical yield will be influenced by the P/O quotient of the process organism. Furthermore, where the formation of a product is linked to the net production of ATP and/or NADH, the P/O quotient will influence the rate of product formation. It follows that to estimate the potential for yield improvement for a given primary or secondary metabolite, it is necessary to determine the P/O quotient of the producing organism. 

We have seen that both the maintenance energy requirement and the P/O quotient of the process micro-organism influences the rate of product formation. In the following sections we will consider how these two factors can be determined, together with the maximum biomass yield. 

In practice, carbon limited chemostat cultures are used to estimate the P/O quotient These conditions are used because they favour the most efficient conversion of the carbon substrate into cellular material, ie the highest efficiency of energy conservation. The steady state respiration rate (qo,) is measured as a function of dilution rate (specific growth rate) and Yq can be obtained from the reciprocal of the slope of the plot. qo, is also known as the metabolic quotient for oxygen or the specific rate of oxygen consumption. 

The P/O quotient obtained in this way is therefore only an approximation. Nevertheless, the values can be used as a comparitive measure of growth efficiency,... 

The extent to which the yield of metabolite can be improved is indicated by the difference between the theoretical and observed yields. The latter must, of course, be corrected for substrates requirements of growth and maintenance. Clearly, the influence of the P/O quotient on the theoretical yield will depend on the relationship between energy and metabolite synthesis. Three classes of metabolite can be distinguished in this respect... 

Metabolites whose biosynthesis is energy requiring, for example exopolysaccharides using certain substrates. Here, part of the substrate has to be oxidised to provide ATP for biosynthesis and thus the P/O quotient of the producing organism influences the theoretical yield. 

Metabolites whose biosynthesis leads to the net production of ATP and/or reducing equivalents, for example organic adds and certain secondary metabolites. In these cases, the P/O quotient influences the extent to which energy can be dissipated. 

The energetic requirements of exopolysaccharide production from various carbon sources can be calculated if the P/O quotient during growth on the carbon substrate is known. Table 3.1 shows molar growth yields measured during carbon limited growth in chemostat culture. 

Now enter this value for P/ O quotient into Table 3.1. 

We can see from Table 3.1 that the P/ O quotient is virtually independent of the carbon constant P/O source. We can therefore assume a constant P/O quotient when calculating the, , olient energetic consequences of exopolysaccharide production from different carbon sources. 

The growth efficiency (Yfilf ) is proportional to the P/O quotient this is expected from the equation used to calculate P/O (section 3.4, E - 3.16). 

In order to quantify the scope for improvement of exopolysaccharide production, it is first necessary to correct the observed yields of exopolysaccharide for the amount of carbon substrate and oxygen required for cell production. The corrected yields are then compared with the theoretical calculated from the P/O quotient for the producing micro-organism. Such a comparison is made in Table 3.3. 

Increase. Increased efficiency of oxidative phosphorylation increases the P/O quotient See E - 3.3. 

The organism, as we have already seen, has a relatively high P/O quotient (high... 

Assuming the P/O-quotient of NADH is 2 and NADPH can be used bioenerge-tically, about 0.5 acetate must be oxidized to neutralize the synthesis. This expenditure of substrate diminishes the product yield coefficient from 0.72 g poly(3HB) per g acetic acid (Table 3) to about 0.57 g per g. Since the experimentally obtained yield coefficient is lower (being, on average, about 0.33 g per g, Table 3), we may draw three conclusions. Firstly, the P/O-quotient is lower than 2. Secondly, the fate of acetate is not strictly determined, i. e., its utilization is not a one-way path and does not terminate in a dead end. Third, there is no doubt that some energy generated from acetate is necessary for homeostasis and turnover processes (maintenance) under conditions of poly(3HB) synthesis and accumulation (with acetic acid as an uncoupler). 

Determine the P/O quotient for the bacterium if Y during growth on glucose... 

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