Substrate Availability

Metathetical reactions of pyrimidines have been studied widely over the last hundred years. Indeed, probably 90% of all known pyrimidines have been made from a relatively few pyrimidine substrates available by primary synthesis. 

Availability of ADP and substrate Availability of substrate only The capacity of the respiratory chain itself, when all substrates and components are present in saturating amounts Availability of ADP only Availability of oxygen only... 

The chemistry described in this review article demonstrates the impressive positive influence that catalytic RCM has had on our research in connection to the development of other catalytic and enantioselective C-C bond forming reactions. There is no doubt that in the absence of pioneering work by Schrock and Grubbs, the Zr-catalyzed alkylation and kinetic resolution would be of less utility in synthesis. The number of unsaturated heterocyclic and carbocyclic substrates available for Zr-catalyzed asymmetric carbomagnesation would be far more limited without catalytic RCM. 

X particulate substrate (hydrolyzable substrate) available as substrate for the microorganisms after hydrolysis by extracellular enzymes and diffusion of the products into the cell... 

Urine and feces represent a small portion of the total substrate available at a waterhole (Merte 2006), yet they are a source of chemical signals. At both study sites, approximately 50% of the elephants observed investigated the general... 

Volkering, F., Breure, A. M., Sterkenburg, A. and van Andel, J. G. (1992). Microbial degradation of polycyclic aromatic hydrocarbons effect of substrate availability on bacterial growth kinetics, Appl. Microbiol. Biotechnol., 36, 548-552. 

Substrate availability is clearly a crucial factor in determining the flux through the pathway. To understand metabolic control fully we will need to apply our knowledge of... 

Substrate availability to the cell is affected by the supply of raw materials from the environment. The plasma membranes of cells incorporate special and often specific transport proteins (translocases) or pores that permit the entry of substrates into the cell interior. Furthermore pathways in eukaryotic cells are often compartmentalized within cytoplasmic organelles by intracellular membranes. Thus we find particular pathways associated with the mitochondria, the lysosomes, the peroxisomes, the endoplasmic reticulum for example. Substrate utilization is limited therefore by its localization at the site of need within the cell and a particular substrate will be effectively concentrated within a particular organelle. The existence of membrane transport mechanisms is crucial in substrate delivery to, and availability at, the site of use. 

Substrate availability for certain reactions can be optimized by anaplerotic ( topping-up ) reactions. For example, citrate synthase is a key control point of the TCA cycle. The co-substrates of citrate synthase are acetyl-CoA and oxaloacetate (OAA) and clearly, restriction in the availability of either substrate will decrease the rate of the citrate synthase reaction. Suppose, for example, a situation arises when acetyl-CoA concentration is significantly higher than that of OAA, the concentration of the latter can be topped-up and the concentration of acetyl-CoA simultaneously reduced by diverting some of the pyruvate away from acetyl-CoA synthesis (via pyruvate dehydrogenase) to OAA synthesis (via pyruvate carboxylase) as shown in Figure 3.1. The net effect is to balance the relative concentrations of the two co-substrates and thus to promote citrate synthase activity. 

The control via activation or inhibition of the rate(s) of an enzyme-catalyzed reaction(s). This control includes the increase or decrease in the stability or half-life of the enzyme(s). There are many different means by which control can be achieved. These include 1. Substrate availability and reaction conditions (e.g., pH, temperature, ionic strength, lipid interface activation) 2. Magnitude of Vraax sud valucs) 3. Activation (particularly, feedforward activation) 4. Isozyme formation 5. Com-partmentalization and channeling 6. Oligomerization/ polymerization 7. Feedback inhibition and cooperativity (particularly, allosterism and/or hysteresis) 8. Covalent modification and 9. Gene regulation (induction repression)... 

B. The rate or flux of substrates through a pathway is also dependent on substrate availability. 

Sandberg, G. Hallmans, and H HV201 Andersson. Substrates available for colonic fermentation from oat, barley and wheat bread diets. A study in ileostomy subjects. Br J Nutr 1996 76(6) HV202... 

BioSolve is a commercially available biodegradable surfactant that is used to enhance bioremediation of petroleum hydrocarbons in soil and water. According to the vendor, BioSolve emulsifies and encapsulates petroleum-based products so that they become nonflammable and more readily bioavailable. Bioavailability is the combination substrate availability and substrate transport that allows for the initiation of bioremediation. 

Enzymes that make NPs necessarily act on complex molecules. Such chemicals can be very hard for chemists to synthesise hence, the range of substrates available to the biochemist to assess substrate specificity was often limited. 

This is a key feature of the system for anyone who wants to understand and rationalize the effects of the microenvironment of a biocatalyst on its activity, its stability, or its specificity. Since for many years the use of thermodynamic activity was recommended for quantifying substrate availability in non-conventional media [17, 18], the replacement of concentrations of species by their thermodynamic activities in liquid non-conventional media requires a knowledge of their activity coefficients (y values). And this point is still far from being straightforward, as (a) values depend on molar ratios of other species present in the medium, and (b) methods used to estimate these values, such as UNI FAC group contribution method [19], are often called into question, and claimed to be sources of inaccuracy [20, 21]. 

DMB reagent 3.2 mg DMB (pure substrate available from Aldrich, cat. no. 341088, Taufkirchen, Germany) is dissolved in 150 ml formiate buffer. The solution can be... 

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