Metabolic flux rate

Deussen A (2000) Metabolic flux rates of adenosine in the heart. Naunyn Schmiedebergs Arch Pharmacol 362(4-5) 351-363... 

Suarez, R.K., J.R.B. Lighton, B. Joos, S.P. Roberts, and J.F. Harrison (1996). Energy metabolism, enzymatic flux capacities, and metabolic flux rates in flying honeybees. Proc. Natl. Acad. Sci. USA 93 12616-12620. 

The -based metabolic flux analysis is a more advanced technique that calculates the metabolic flux vector r by additionally using the -labelled pattern of the stable and abundant protein-bound amino acids determined by either gas chromatography coupled with mass spectroscopy (GC/MS) and/or nuclear magnetic resonance (NMR) (Christensen et al. 2002 Sauer 2006 Wiechert et al. 2(X)1 Zamboni et al. 2005, 2009). The most sophisticated technique known as kinetic flux profiling has recently been developed to calculate the metabolic flux vector r by measuring the dynamic incorporation of labeled substrates (e.g. C, N) into downstream intermediate metabolites. This measurement can be subsequently used to calculate metabolic fluxes (rates) directly without relying on the simplified metabolic network like the traditional MFA approach (Yuan et al. 2006, 2008,2010). 

The first example of a dynamic flux analysis was a study performed in the 1960s [269]. In the yeast Candida utilis, the authors determined metabolic fluxes via the amino acid synthesis network by applying a pulse with 15N-labeled ammonia and chasing the label with unlabeled ammonia. Differential equations were then used to calculate the isotope abundance of intermediates in these pathways, with unknown rate values fitted to experimental data. In this way, the authors could show that only glutamic acid and glutamine-amide receive their nitrogen atoms directly from ammonia, to then pass it on to the other amino acids. 

Metabolism and the individual reactions which comprise a pathway represent a dynamic process. Terms such as flow , substrate flux , rate and turnover are all used to communicate the idea of the dynamic nature of metabolism. 

Metabolic flux this is defined as the amount of the unit of interest, usually the mass of a metabolite in moles (often micromoles, rather) per unit time per unit area or volume, or often grams dry cell weight (dew) [pmol (h g dew)-1] passing between components A and B of the metabolic system. A metabolic rate is equivalent dimensionally to a specific reaction rate. Metabolic flux is the basic unit of observation and modeling in metabolic engineering. 

Components are commonly represented as nodes in the metabolic network. These nodes can act as branch points if the number of input and output fluxes is not equivalent. Non-essential reactions around a node can be collected into reaction groups the coefficients of their fluxes, in general termed the metabolic flux coefficients (in analogy to rate coefficients), can be rearranged as group control coefficients. 

Macromolecular biomass composition is of obvious interest when the biomass itself is the product, such as algal biomass in [18], or for production of singlecell protein, for e.g. animal feedstock. Moreover, for a precise metabolic flux analysis, changes in biomass composition should be taken in account. For example, Henriksen et al. [19] observed with E. coli under different growth rates, that the levels of DNA and lipids were relatively constant, whereas the proteins and stable RNA levels increased with the specific growth rate and the total amount of carbohydrates decreased. 

Like the examples mentioned above, most examples of metabolic flux analysis by metabolite balancing have redox balances as a central constraint used in the determination of the flux distribution. However, the redox balance is, especially under aerobic conditions, subject to uncertainties which make it less suitable for estimation of the fluxes. Part of the reason for this is to be found in futile cycles, e. g., oxidation of sulfides to disulfides, where reductive power is needed to reduce the disulfides. The net result of this reaction is reduction of molecular oxygen to water, and oxidation of NADPH to NADP+. Since the consumption rate of oxygen of these specific reactions is impossible to measure, the result may be that the NADPH consumption is underestimated. This is in accordance with the finding that when the NADPH-producing reactions are estimated independently of the NADPH-consuming reactions, there is usually a large excess of NADPH that needs to be oxidized by reactions not included in the network, e. g., futile cycles [11-13]. 

It is thus apparent that the utilization of tryptophan as a precursor for NAD synthesis depends on both the amount of tryptophan to be metabolized and also the rate of metabolic flux through the pathway. The activities of three... 

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