Rate-limiting enzyme

X. ARE OTHER STARCH BIOSYNTHETIC ENZYMES RATE LIMITING ... 

A6-desaturase enzyme rate limiting and affected by many factors... 

A. (The gas phase estimate is about 100 picoseconds for A at 1 atm pressure.) This suggests tliat tire great majority of fast bimolecular processes, e.g., ionic associations, acid-base reactions, metal complexations and ligand-enzyme binding reactions, as well as many slower reactions that are rate limited by a transition state barrier can be conveniently studied with fast transient metliods. 

Eor measurement of a substrate by a kinetic method, the substrate concentration should be rate-limiting and should not be much higher than the enzyme s K. On the other hand, when measuring enzyme activity, the enzyme concentration should be rate-limiting, and consequentiy high substrate concentrations are used (see Catalysis). 

Assay of Enzymes In body fluids, enzyme levels aie measured to help in diagnosis and for monitoiing treatment of disease. Some enzymes or isoenzymes are predominant only in a particular tissue. When such tissues are damaged because of a disease, these enzymes or isoenzymes are Hberated and there is an increase in the level of the enzyme in the semm. Enzyme levels are deterrnined by the kinetic methods described, ie, the assays are set up so that the enzyme concentration is rate-limiting. The continuous flow analyzers, introduced in the early 1960s, solved the problem of the high workload of clinical laboratories. In this method, reaction velocity is measured rapidly the change in absorbance may be very small, but within the capabiUty of advanced kinetic analyzers. 

The neurotransmitter must be present in presynaptic nerve terminals and the precursors and enzymes necessary for its synthesis must be present in the neuron. For example, ACh is stored in vesicles specifically in cholinergic nerve terminals. It is synthesized from choline and acetyl-coenzyme A (acetyl-CoA) by the enzyme, choline acetyltransferase. Choline is taken up by a high affinity transporter specific to cholinergic nerve terminals. Choline uptake appears to be the rate-limiting step in ACh synthesis, and is regulated to keep pace with demands for the neurotransmitter. Dopamine [51 -61-6] (2) is synthesized from tyrosine by tyrosine hydroxylase, which converts tyrosine to L-dopa (3,4-dihydroxy-L-phenylalanine) (3), and dopa decarboxylase, which converts L-dopa to dopamine. 

The dopamine is then concentrated in storage vesicles via an ATP-dependent process. Here the rate-limiting step appears not to be precursor uptake, under normal conditions, but tyrosine hydroxylase activity. This is regulated by protein phosphorylation and by de novo enzyme synthesis. The enzyme requites oxygen, ferrous iron, and tetrahydrobiopterin (BH. The enzymatic conversion of the precursor to the active agent and its subsequent storage in a vesicle are energy-dependent processes. 

An important dmg in the regulation of cholesterol metaboHsm is lovastatin [75330-75-5] which is an HMG—CoA reductase inhibitor (see Cardiovascularagents). p-Hydroxy-p-methyl glutarate—coenzyme A (HMG—CoA) reductase is the rate-limiting enzyme of cholesterol synthesis. Lovastatin is actually a prodmg, which is eventually hydrolyzed in the Hver to its active, P-hydroxylated form (5). 

Hydrolysis of esters and amides by enzymes that form acyl enzyme intermediates is similar in mechanism but different in rate-limiting steps. Whereas formation of the acyl enzyme intermediate is a rate-limiting step for amide hydrolysis, it is the deacylation step that determines the rate of ester hydrolysis. This difference allows elimination of the undesirable amidase activity that is responsible for secondary hydrolysis without affecting the rate of synthesis. Addition of an appropriate cosolvent such as acetonitrile, DMF, or dioxane can selectively eliminate undesirable amidase activity (128). 

Catecholamine biosynthesis begins with the uptake of the amino acid tyrosine into the sympathetic neuronal cytoplasm, and conversion to DOPA by tyrosine hydroxylase. This enzyme is highly localized to the adrenal medulla, sympathetic nerves, and central adrenergic and dopaminergic nerves. Tyrosine hydroxylase activity is subject to feedback inhibition by its products DOPA, NE, and DA, and is the rate-limiting step in catecholamine synthesis the enzyme can be blocked by the competitive inhibitor a-methyl-/)-tyrosine (31). 

Enzymes assist formation of proper disulfide bonds during folding Isomerization of proline residues can be a rate-limiting step in protein folding Proteins can fold or unfold inside chaperonins GroEL is a cylindrical structure with a... 

In zero-orrler kinetics, a constant amount of a chemical compound is excreted per unit of rime. In most cases, this phenomenon is caused by the saturation of a rate-limiting enzyme, and the enzyme commonly functions at its maximal rate, i.e., a constant amount of a chemical compound is metabolized per unit time. A good example is ethyl alcohol alcohol dehydrogenase becomes saturated at relatively low concentrations. Because of this saturation, ethyl alcohol is eliminated at a constant rate about 7 g/h. However, rhe reason is not always an enzyme anv... 

FIGURE 16.21 Burst kinetics observed iu the chymotrypsiii reaction. A burst of nitrophe-nolate production is followed by a slower, steady-state release. After an initial lag period, acetate release is also observed. This kinetic pattern is consistent with rapid formation of an acyl-enzyme intermediate (and the burst of nitrophenolate). The slower, steady-state release of products corresponds to rate-limiting breakdown of the acyl-enzyme intermediate. 

D-Methylmalonyl-CoA, the product of this reaction, is converted to the L-isomer by methylmalonyl-CoA epunerase (Figure 24.19). (This enzyme has often and incorrectly been called methylmalonyl-CoA racemase. It is not a racemase because the CoA moiety contains five other asymmetric centers.) The epimerase reaction also appears to involve a carbanion at the a-position (Figure 24.20). The reaction is readily reversible and involves a reversible dissociation of the acidic a-proton. The L-isomer is the substrate for methylmalonyl-CoA mutase. Methylmalonyl-CoA epimerase is an impressive catalyst. The for the proton that must dissociate to initiate this reaction is approximately 21 If binding of a proton to the a-anion is diffusion-limited, with = 10 M sec then the initial proton dissociation must be rate-limiting, and the rate constant must be... 

Acetylcholine serves as a neurotransmitter. Removal of acetylcholine within the time limits of the synaptic transmission is accomplished by acetylcholinesterase (AChE). The time required for hydrolysis of acetylcholine at the neuromuscular junction is less than a millisecond (turnover time is 150 ps) such that one molecule of AChE can hydrolyze 6 105 acetylcholine molecules per minute. The Km of AChE for acetylcholine is approximately 50-100 pM. AChE is one of the most efficient enzymes known. It works at a rate close to catalytic perfection where substrate diffusion becomes rate limiting. AChE is expressed in cholinergic neurons and muscle cells where it is found attached to the outer surface of the cell membrane. 

Rate-limiting enzyme in cholesterol biosynthesis inhibition by statins results in reduction of plasma LDL-cholesterol levels. 

When induced in macrophages, iNOS produces large amounts of NO which represents a major cytotoxic principle of those cells. Due to its affinity to protein-bound iron, NO can inhibit a number of key enzymes that contain iron in their catalytic centers. These include ribonucleotide reductase (rate-limiting in DNA replication), iron-sulfur cluster-dependent enzymes (complex I and II) involved in mitochondrial electron transport and cis-aconitase in the citric acid cycle. In addition, higher concentrations of NO,... 

A class of cholesterol lowering drugs that inhibit 3-hydroxy-3-methylglutary 1-CoA reductase, the rate-limiting enzyme step in cholesterol biosynthesis. 

Tyrosine hydroxylase (TH) is an enzyme that catalyzes the hydroxylation of tyrosine to 3,4-dihydroxypheny-lalanine in the brain and adrenal glands. TH is the rate-limiting enzyme in the biosynthesis of dopamine. This non-heme iron-dependent monoxygenase requires the presence of the cofactor tetrahydrobiopterin to maintain the metal in its ferrous state. 

See also Enzyme cofactors downhill trajectories for, 196,197 mechanism of catalytic reaction, 190-192 metal substitution, 200-204 potential surfaces for, 192-195,197 rate-limiting step of, 190 reference solution reaction for, 192-195,... 

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