Enzymes rule over in biochemical processes. Enzyme Commission (EC) numbers have been proposed in ord — Organic Chemistry Chemistry Question
Number One Enzyme
Enzymes rule over in biochemical processes. Enzyme Commission (EC) numbers have been proposed in order not to be lost in the vast variety of biocatalysts. This is a four-level classification based on the types of enzymatic reactions, each specific reaction being described by four numbers separated by periods. Number 1 of 6 top-level EC numbers is attributed to oxidoreductases, enzymes catalyzing oxidation/reduction reactions (transfer of H and O atoms or electrons from one substance to another). These reactions are of a great importance for both energy transformation and xenobiotic metabolism.
Changes of the oxidation state of a molecule can be achieved in different ways, which makes possible a more detailed classification of oxidoreductases. Many of these require a cofactor or coenzyme. Thus, dehydrogenases remove two hydrogen atoms from an organic substrate and pass them over to a suitable acceptor like nicotinamide adenine dinucleotide {phosphate} (NAD+ {NADP+}) or flavin adenine dinucleotide (FAD).
adenine adenine adenine flavin
ribose –– P –– P –– ribose ribose –– P –– P –– ribitol
[P]
Write down the equation of ethanol oxidation involving NAD+ as the coenzyme. Show the changes in the nicotinamide moiety.
Model Answer
Reaction: CH3CH2OH + NAD+ = CH3CHO + NADH + H+.
Only nicotinamide moiety of the coenzyme is affected as a result of the reaction:
[VISUAL]
Sometimes the route of dehydrogenase reaction is determined by the coenzyme/cofactor involved. Predict the structures of A1–A3 in the following scheme if it is known that 1.00 g of A3 gives 7.52 g of silver precipitate under Tollens` reagent treatment. [VISUAL]
Model Answer
Both enzymatic steps are oxidation (dehydrogenation) reactions. Generally, coenzymes reveal a “specialization”. Thus, the NAD-system is involved in dehydrogenation of polar bonds (discussed above with ethanol as an example), whereas the FAD-system in that of nonpolar bonds. Therefore, it is necessary to decide on the A3 nature. Its equivalent in the silver mirror reaction is 1.00 * 107.9 / 7.52 = 14.3 g mol–1. Usually oxidation of an aldehyde group provides 2 electrons. Then such a small equivalent can correspond to low molecular weight fragment only. Analyzing cleavage options, one comes to the following scheme (note, that intermediates of A3 oxidation are easily decarboxylated): [VISUAL]
Oxygenases catalyze direct addition of oxygen atoms originating from O2 to an organic substrate. These enzymes are further subdivided into mono- and di-oxygenases depending on the number of incorporated atoms (one and two, respectively). In the case of monooxygenases the other oxygen atom is reduced to water by an appropriate reactant. Cytochrome P-450-dependent monooxygenase is an exceptionally important representative of the class.
An aromatic compound B1 contains 77.75 % C and 14.80 % O by mass. B1 metabolism in rat liver microsomes develops in two ways (see on the hereunder scheme). [VISUAL]
B2 reveals 4 signals in 1H-NMR spectrum. B2 and B3 undergo color changes when added to 1% FeCl3 solution, whereas the B1 color is unaffected by such treatment. All of B1 to B4 are composed of the same elements. B4 is a gas under STP.
Decipher B1 to B4. Write down the balanced equations.
Model Answer
The rest of B1 mass is attributed to hydrogen. Then C : H : O = 77.75/12.01 : (100 - 77.75 - 14.80)/1.008 : 14.80/16.00 = 7 : 8 : 1. It is anisole (C7H8O) belonging to aromatics, still denying the probe for phenols. Monooxygenase reaction should lead to a product C7H8O2, which, in turn, contains a phenolic hydroxyl. Among possible aromatic hydroxylation products, only 4-methoxyphenol (B2) contains the desired number of signals in the NMR spectrum. Intermediate B (C7H8O2) spontaneously decomposes into the products of the same qualitative composition, which allows us identifying phenol (positive FeCl3 test of B3) and formaldehyde (B4 is a gas at STP). The overall equations for the two directions of the oxidative metabolism (do not forget, that the cofactor is an acceptor of the second oxygen atom) are:
C7H8O + O2 + NADPH + H+ = C7H8O2 + NADP+ + H2O;
C7H8O + O2 + NADPH + H+ = C6H6O + CH2O + NADP+ + H2O.
Under oxygen deficiency B1 partially converts into B5, an isomer of B2. This transformation requires a vitamin as the oxygen acceptor. Further oxidation of B5 leads to B6 characterized by 2 signals in 1H-NMR spectrum.
Draw the structures of B5 and B6 as well as of both forms of the vitamin.
Model Answer
Careful consideration of the transformations allows us concluding that B5 and B6 have the composition of C7H8O2 and C7H6O2, respectively. So, B5 can be either meta- or ortho- methoxyphenol. Opting for the latter is based on the high symmetry of the product B6, which can only be methylenedioxybenzene. Finally, ascorbic acid is the electron acceptor: [VISUAL]
Like dehydrogenases, oxidases remove hydrogen atoms from an organic substrate transferring these either to the molecular oxygen or X, which finally results in water formation in both cases. The phenol oxidation can be described by the following equation:
2 Phenol + X = Y + 2 H2O
The reaction develops via a free radical intermediate and leads to a mixture of isomers of Y (77.4 % C by mass).
Decipher X. Draw the isomers of Y that show a sole peak in the phenolic region of 1H-NMR spectra at low temperatures.
Model Answer
The reaction equation suggests that Y contains 12 carbon atoms with the molar mass of 12.01 * 12 / 0.774 = 186 g mol–1. With an account for the oxidative nature of the conversion, we get a unique meaningful solution for the equation in integers: Y = C12H10O2. Checking the balance one finds that Х = H2O2. So, it is a peroxidase reaction involving a phenolic free radical generation with the unpaired electron delocalized between the oxygen atom and the ortho / para ring positions. Accordingly, Y represents the dimer of such particles. Spectral data correspond to the following substances: [VISUAL]