IOM 2017, Теория — Задача 4. How do organisms obtain energy?

2nd Olympiad of Metropolises — Chemistry, Theoretical Problems, Official English version · 6 септември 2017 г.

Условие

Problem 4. How do organisms obtain energy? (10 marks)

Many living things obtain energy by oxidizing organic molecules with reduced and/or partially oxidized carbon atoms. A substance in a more reduced state provides for greater energy that can be potentially obtained from it. Some carbon containing compounds are given hereunder in an arbitrary sequence: $\mathrm{CO,CH_4,C_2H_5OH,HCOOH,CO_2,CH_3OH,C_6H_{12}O_6,C,HCHO,CH_3CHO}$.

1 Arrange the compounds in the increasing order (from the most reduced to the most oxidized) of the averaged oxidation degree (oxidation number) of its carbon atoms. If two or more compounds are equal in this parameter, write down the corresponding molecules one below the other.

[2 т.]

Being the initial stage of digestion of monosaccharides, glycolysis occurs under both aerobic (with oxygen) and anaerobic (without oxygen) conditions leading to one and the same product (pyruvate, the pyruvic acid anion) in both cases. Further direction of pyruvate transformation depends on particular organism and conditions. Metabolic pathways of pyruvate are extremely diverse in anaerobic bacteria. These processes referred to as fermentations allow accumulating energy in the form of ATP as a result of oxygen free oxidation of organic substrates. Apart from widely known lactic acid and alcoholic fermentations, there are other fermentation types leading to organic products with 1 to 4 carbon atoms as well as to mixtures of these products. Some fermentation pathways will be considered below.

A process typical of Escherichia and Salmonella genera is shown on the hereunder scheme. It starts with the lyase (the enzyme facilitating a C–C bond cleavage) catalyzed transformation of pyruvate into acetyl-coenzyme A and the compound D. In some bacterial species, D turns out to be the final fermentation product; whereas other species degrade it to a mixture of equimolar amounts of two gases E and F (E is more than 15 times lighter than F). Acetyl-coenzyme A affords two asymmetric compounds G and H containing two carbon atoms each. Only one carbon atom in each of G and H is linked with oxygen atom(s). The averaged oxidation degree of carbon atoms in G is higher than that in H. Under definite conditions, G and H interact at 1:1 molar ratio leading to the compound I with a specific strong odor. I is free of asymmetric carbon atoms.

2 Draw the compounds D-I.

[7 т.]

Another type of fermentation is met in Clostridium genus. In this case one molecule of glucose affords a mixture of the same gases E and F (in the same ratio as in i. 2), as well as an unbranched anion J composed of 3 elements. The corresponding acid J1 contains 54.52 % by mass of C.

3 Write down the equation of glucose fermentation into E, F, and J1, occurred in Clostridium bacteria.

Calculations and work: ______

Equation: ______ [3 т.]

P, Q, and T composed of 4 carbon atoms each are formed as a result of glucose fermentation by bacteria belonging to Enterobacter genus (see the rightwards scheme).

Information about the enzymes on the scheme:

$\mathrm E_1$ is $\alpha$-acetolactate synthase,

$\mathrm E_2$ is $\alpha$-acetolactate decarboxylase,

$\mathrm E_3$ and $\mathrm E_4$ belong to same class of enzymes.

There is one peak in the $^1\mathrm H$ NMR spectrum of P, and four peaks of different intensity in that of Q. Q contains one asymmetric center. T1-T3 are stereoisomers. Transformation of P into Q occurs in both directions in some bacteria.

4 What class of enzymes do $\mathrm E_3$ and $\mathrm E_4$ belong to? Tick the appropriate box.

• ☐ Decarboxylase
• ☐ Carboxylase
• ☐ Oxidoreductase
• ☐ Lipase
• ☐ Synthase [2 т.]

5 Draw the structural formulae of P, Q, and T. Indicate the absolute configuration (writing R or S) of each stereocenter in T1-T3.

[10 т.]

Compounds other than products of carbohydrate digestion can be involved in fermentation. A redox reaction between a pair of amino acids referred to as Stickland fermentation is found in bacteria Pseudomonas. The leftwards scheme illustrates the Stickland reaction between two canonical $\alpha$-amino acids X and Y.

6 Draw the structural formulae of X-X2 and Y.

[6 т.]

Designations used in the text:

CoA-SH is co-enzyme A,

ADP and ATP are adenosine diphosphate and adenosine triphosphate, respectively,

$\mathrm P_i$ is inorganic phosphate.

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