Enzyme catalyzed

This poster indicates the structures of the compounds involved in a reaction, the enzymes catalyzing a reaction, the coenzymes and regulators involved, and whether such a reaction is a general pathway occurring in all species, or a pathway specific to higher plants, animals, or unicellular organisms. 

Many reactions classified as dehydrogenations occur within the cells of living sys terns at 25°C H2 is not one of the products however Instead the hydrogens are lost m separate steps of an enzyme catalyzed process The enzyme indicated m the reaction... 

The biological dehydrogenation of succinic acid described in Section 5 8 is 100% stereoselective Only fumaric acid which has a trans double bond is formed High levels of stereoselectivity are characteristic of enzyme catalyzed reactions... 

Optically inactive starting materials can give optically active products only if they are treated with an optically active reagent or if the reaction is catalyzed by an optically active substance The best examples are found m biochemical processes Most bio chemical reactions are catalyzed by enzymes Enzymes are chiral and enantiomerically homogeneous they provide an asymmetric environment m which chemical reaction can take place Ordinarily enzyme catalyzed reactions occur with such a high level of stereo selectivity that one enantiomer of a substance is formed exclusively even when the sub strate is achiral The enzyme fumarase for example catalyzes hydration of the double bond of fumaric acid to malic acid m apples and other fruits Only the S enantiomer of malic acid is formed m this reaction... 

AN ENZYME-CATALYZED NUCLEOPHILIC SUBSTITUTION OF AN ALKYL HALIDE... 

Many naturally occurring substances are epoxides You have seen two examples of such compounds already m disparlure the sex attractant of the gypsy moth (Section 6 18) and m the carcinogenic epoxydiol formed from benzo[a]pyrene (Section 118) In most cases epoxides are biosynthesized by the enzyme catalyzed transfer of one of the oxy gen atoms of an O2 molecule to an alkene Because only one of the atoms of O2 is trans ferred to the substrate the enzymes that catalyze such transfers are classified as monooxy genases A biological reducing agent usually the coenzyme NADH (Section 15 11) is required as well... 

An example of a biologically important aide hyde is pyridoxal phosphate which is the active form of vitamin Bg and a coenzyme for many of the reac tions of a ammo acids In these reactions the ammo acid binds to the coenzyme by reacting with it to form an imine of the kind shown in the equation Re actions then take place at the ammo acid portion of the imine modifying the ammo acid In the last step enzyme catalyzed hydrolysis cleaves the imme to pyridoxal and the modified ammo acid... 

Enzyme catalyzed reductions of carbonyl groups are more often than not com pletely stereoselective Pyruvic acid for example is converted exclusively to (5) (+) lactic acid by the lactate dehydrogenase NADH system (Section 15 11) The enantiomer... 

Phenolic compounds are commonplace natural products Figure 24 2 presents a sampling of some naturally occurring phenols Phenolic natural products can arise by a number of different biosynthetic pathways In animals aromatic rings are hydroxylated by way of arene oxide intermediates formed by the enzyme catalyzed reaction between an aromatic ring and molecular oxygen... 

Cyanogemc glycosides are potentially toxic because they liberate hydrogen cyanide on enzyme catalyzed or acidic hydrolysis Give a mechanistic explanation for this behavior for the specific cases of... 

We 11 see numerous examples of both reaction types m the following sections Keep m mind that m vivo reactions (reactions m living systems) are enzyme catalyzed and occur at far greater rates than those for the same transformations carried out m vitro ( m glass ) m the absence of enzymes In spite of the rapidity with which enzyme catalyzed reactions take place the nature of these transformations is essentially the same as the fundamental processes of organic chemistry described throughout this text... 

Before leaving this biosynthetic scheme notice that PGE2 has four chirality cen ters Even though arachidomc acid is achiral only the stereoisomer shown m the equa tion IS formed Moreover it is formed as a single enantiomer The stereochemistry is controlled by the interaction of the substrate with the enzymes that act on it Enzymes offer a chiral environment m which biochemical transformations occur and enzyme catalyzed reactions almost always lead to a single stereoisomer Many more examples will be seen m this chapter... 

Isopentenyl pyrophosphate undergoes an enzyme catalyzed reaction that converts It m an equilibrium process to 3 methyl 2 butenyl pyrophosphate (dimethylallyl pyrophosphate)... 

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

FIGURE 26 10 The biosyn thetic conversion of squa lene to cholesterol proceeds through lanosterol Lano sterol IS formed by enzyme catalyzed cyclization of the 2 3 epoxide of squalene... 

Ammonia reacts with the ketone carbonyl group to give an mine (C=NH) which is then reduced to the amine function of the a ammo acid Both mine formation and reduc tion are enzyme catalyzed The reduced form of nicotinamide adenine diphosphonu cleotide (NADPH) is a coenzyme and acts as a reducing agent The step m which the mine is reduced is the one m which the chirality center is introduced and gives only L glutamic acid... 

Step 2 Enzyme catalyzed proton transfer steps cause migration of the double bond converting the mine formed m step 1 to an isomeric mine... 

FIGURE 27 4 The mecha nism of transamination All the steps are enzyme catalyzed... 

FIGURE 27 5 Tyrosine is the biosynthetic precursor to a number of neurotransmit ters Each transformation IS enzyme catalyzed Hydroxy lation of the aromatic ring of tyrosine converts it to 3 4 dihyd roxyphenylalanine (l dopa) decarboxylation of which gives dopamine Hy droxylation of the benzylic carbon of dopamine con verts It to norepinephrine (noradrenaline) and methy lation of the ammo group of norepinephrine yields epi nephrine (adrenaline)... 

Knowing how the protein chain is folded is a key ingredient m understanding the mechanism by which an enzyme catalyzes a reaction Take carboxypeptidase A for exam pie This enzyme catalyzes the hydrolysis of the peptide bond at the C terminus It is... 

The imidazole nng of the histidine side chain acts as a proton acceptor in certain enzyme catalyzed reactions Which is the more stable protonated form of the histidine residue A or Why" ... 

An Enzyme-Catalyzed Nucleophilic Substitution of an Alkyl Halide... 

Enzyme-Catalyzed Reactions Enzymes are highly specific catalysts for biochemical reactions, with each enzyme showing a selectivity for a single reactant, or substrate. For example, acetylcholinesterase is an enzyme that catalyzes the decomposition of the neurotransmitter acetylcholine to choline and acetic acid. Many enzyme-substrate reactions follow a simple mechanism consisting of the initial formation of an enzyme-substrate complex, ES, which subsequently decomposes to form product, releasing the enzyme to react again. 

Determining and for Enzyme-Catalyzed Reactions The value of Vmax and... 

Perez-Bendito, D. Silva, M. Kinetic Methods in Analytical Chemistry. Ellis Horwood Chichester, England, 1988. Additional information on the kinetics of enzyme catalyzed reactions maybe found in the following texts. 

Pisakiewicz, D. Kinetics of Chemical and Enzyme-Catalyzed Reactions. Oxford University Press New York, 1977. 

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