Enzyme-catalyzed steps

The final product(s) generated by a sequence of enzyme-catalyzed steps within a metabolic pathway. End products frequently act as potent inhibitors of the first commited step in a metabolic pathway leading to their synthesis. End products are also known to accelerate the rate of another metabolic pathway leading to a different set of end products. The highly coordinated biosynthesis of adenine and guanine nucleotides provides examples of negative and positive effects of end products of each pathway. 

Ethanol is ordinarily metabolized in the liver by oxidation in two enzyme-catalyzed steps to acetaldehyde and ultimately acetate. 

D. Conversion of pyruvate to PEP requires two enzyme-catalyzed steps. 

FIGURE 22-17 Biosynthesis of tryptophan from chorismate in bacteria and plants. In E. coli, enzymes catalyzing steps (T) and are subunits of a single complex. 

The limits between the areas are blurred biotransformations and enzyme catalyses with cmde extracts or pure enzymes are often summarized under the term biocatalysis . Biocatalytic processes are taken to mean transformations of a defined substrate to a defined target product with one or several enzyme-catalyzed steps. 

A metabolic pathway consists of a sequence of enzyme-catalyzed steps. The pathway usually has a rate-limiting step, the reaction in the sequence with the lowest velocity. This may be due to the enzyme having a high Km for its substrate or to the enzyme being subject to inhibition by a negative effector, usually a product of the pathway. In the latter case, an allosteric enzyme is involved. 

Whereas tetrahydrobiopterin is biosynthesized from GTP via just three enzyme-catalyzed steps (2), some coenzyme biosynthetic pathways are characterized by enormous complexity. Thus, the biosynthesis of vitamin B12 requires five enzymes for the biosynthesis of the precursor uroporhyrinogen III (16) from succinyl-CoA (10) and glycine (11) that is then converted into vitamin B12 via the sequential action of about 20 enzymes (3). Additional enzymes are involved in the synthesis of the building blocks aminopropanol and dimethylbenzimidazole (4, 5). Vitamin B12 from nutritional sources must then be converted to coenzyme B12 by mammalian enzymes. Ultimately, however, coenzyme B12 is used in humans by only two enzymes, albeit of vital importance, which are involved in fatty acid and amino acid metabolism (6). Notably, because plants do not generate corrinoids, animals depend on bacteria for their supply of vitamin B12 (which may be obtained in recycled form via nutrients such as milk and meat) (7). 

The mevalonate pathway starts with a sequence of two Claisen condensations that afford (6 )-3-hydroxy-3-methyl-glutaryl-CoA (84) from three acetyl-CoA moieties. The pathway affords IPP that can be converted into DMAPP by isomerization. The first committed intermediate of the nonmevalonate pathway is 2C-methyl-D-erythritol 4-phosphate (90) obtained from 1-deoxy-D-xylulose 5-phosphate (43), which is a compound also involved in the biosynthesis of vitamins Bi (46, cf. Fig. 4) and Be (39, cf. Fig. 5), by rearrangement and subsequent reduction. Three enzyme-catalyzed steps are required to convert the compound into the cognate cyclic diphosphate 91 that is then converted reductively into a mixture of IPP and DMAPP by the consecutive action of two iron/sulfur proteins. 

Family resemblance. Propose mechanisms for the two enzymes catalyzing steps in glycogen debranching based on their potential membership in the a-amylase family. 

Pyruvic acid can be converted, in one enzyme-catalyzed step, to all of the following compounds except ... 

The AG of a reaction depends only on the free energy of the products (the final state) minus the free energy of the reactants (the initial state). The AG of a reaction is independent of the path (or molecular mechanism) of the transformation. The mechanism of a reaction has no effect on AG. For example, the AG for the oxidation of glucose to CO and H2O is the same whether it occurs by combustion or by a series of enzyme-catalyzed steps in a cell. 

The same quantity of heat is released whether the sugar is burnt in the air or oxidized in a series of enzyme-catalyzed steps in your body. 

In order to achieve a dynamic kinetic resolution of alcohols, procedures need to be found for the in situ racemization of these substrates. The racemization conditions need to be compatible with the enzyme-catalyzed step, and the product must be inert to racemization. 

The role of enzymes in regulating biochemical reactions makes them an important target in medicinal chemistry and drug research. If a biochemical pathway runs out of control, it may sometimes be regulated by changing the turnover rate of an enzyme-catalyzed step in the pathway through (partial) inhibition of the enzyme. Once the kinetical, chemical, and structural details of an enzyme mechanism are understood, efficient inhibitors can be designed. However, quite different mechanisms of inhibition are possible. 

However, the actual process is extremely complex 13 enzyme-catalyzed steps are required to incorporate each molecule of CO2, so the six CO2 molecules incorporated to form one molecule of C6H12O6 require six repetitions of the pathway. Melvin Calvin and his coworkers took seven years to determine the pathway, using in CO2 as the tracer and painstakingly separating the products formed after different times of light exposure. Calvin won the Nobel Prize in chemistry in 1961 for this remarkable achievement. 

Metabolism is the study of the conversion of biological molecules, especially small molecules, from one to another — for example, the conversion of sugar into carbon dioxide and water, or the conversion of fats into cholesterol. Metabolic biochemists are particularly interested in the individual enzyme-catalyzed steps of an overall sequence of reactions (called a pathway) that leads from one substance to another. 

Glucose 6-phosphate has one of two major fates either glycogenesis or glycolysis. The particular fate is determined by regulation of the fluxes via the enzymes in the two pathways. Even though both of these pathways contain multiple enzyme-catalyzed steps, each pathway has specific controls on the constituent enzymes. 

The glycolytic pathway down to pyruvate involves 10 enzyme-catalyzed steps with three kinases that use ATP (Fig. 11-32). The reaction catalyzed by phosphofructokinase (PFK) plays the major part in controlling the overall flux of the whole pathway. It catalyzes the phosphorylation of fructose 6-phosphate (Fru6P) to fructose 1,6-bisphosphate (Frul6P2). Hexokinase and pyruvate kinase (PK) have relatively higher maximal velocities. PFK is the target for more effector molecules than the other two kinases (see Fig. 11-14). 

Thus, the kinetics of conversions In metabolic cellular sequences, and even in whole cell kinetics, at or near steady state may be expected to resemble the kinetic rate form appropriate to one or a very small number of sequential enzyme catalyzed steps. The implications of this point in kinetic models of structured cell systems are reflected in later contributions in this conference. 

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