TEXT 8. THE PROBLEM OF THE SYNTHESIS OF PROTEINS

The possibility of preparing proteins artificially has long commanded the attention of scientists. Why is it that it has still not been possible to synthesize natural proteins in the laboratory? After all, many other high-molecular compounds are produced synthetically. The reason for this lies in the extremely complicated structure of the protein molecule. When an ordinary polymer, such as capron, is synthesized, identical units of aminocaproic acid link up to form the macromolecule. It is therefore immaterial in what order they link up. Protein molecules, however, consist of different amino acids, and every protein is characterized by a strict order in which the units follow one another. Should as much as one amino acid molecule be in the wrong position, the required protein will not be formed. It is very difficult to achieve such a strict sequence of amino acids when there is a vast number of units in the molecule. Even establishing such a sequence of amino acids is a very complicated matter; only in recent years has it proved possible to decipher it in some proteins.

Furthermore, it is not only the sequence in which the amino acids follow one another that makes the synthesis of proteins such a difficult problem. Every protein molecule has a definite spatial configuration. Often, for instance, they are twisted into a spiral, which in turn can be curled into ball, etc. A synthesized molecule must therefore also have a certain spatial structure.

It is interesting that in a protein molecule twisted into a spiral or curled into a ball the radicals R of the amino acid units (see general formula of amino acids) are largely turned outwards. In these radicals, as may be seen even from the cases of some of the amino acids, there are various functional groups, capable of entering into numerous chemical reactions. It is this that is responsible for the high reactivity of the proteins and for the diversity of their functions in the organism.

Scientists are now probing deep into the mechanism of protein synthesis in organisms and into the chemical processes that take place in protein molecules when they discharge their various vital functions.

The headway made in gaining an insight into the structure and synthesis of proteins has brought scientists face to face with the problem of preparing relatively simple proteins artificially. Beyond doubt, proteins of a more complex structure will also be synthesized in due course.

An understanding of the essence of the complex processes involving proteins as the carriers of life and mastery of the laws governing these processes enhances man's control over nature. For technology too this means fundamentally new processes, which in the past took place only in living organisms. Unprecedented resources are becoming available for producing various substances and materials for public consumption and technology. There will also be new, inexhaustible sources of power, generated by chemical reactions. The task of stamping out disease and extending the human lifespan will become quite realistic.

Notes on the text:

1) to command the attention – овладеть вниманием

2) immaterial – несущественный, неважный

3) to probe deep into – глубоко проникнуть внутрь

4) headway – продвижение вперед, успех

5) to gain insight into – заглянуть, зд. изучать

6) to bring smb. face to face – столкнуть

7) in due course – в свое время

8) to stamp out disease – покончить с болезнями

9) lifespan – продолжительность жизни.

Задания к тексту:

Прочтите 1 абзац текста. Выпишите слова или словосочетания, указывающие на основные этапы развития мысли.

Сократите 1 абзац текста, сохраняя его основную мысль.

Найдите в 1 абзаце текста условное предложение. Определите тип условного предложения и переведите его на русский язык.

4. Прочтите 2 и 3 абзацы текста. Расположите следующие предложения в последовательности, соответствующей содержанию прочитанного:

1. Protein molecules are twisted into a spiral.

2. A synthesized molecule must therefore also have a certain spatial structure.

3. Every protein molecule has a definite spatial configuration.

4. A spiral can be curled into a ball.

5. Functional groups are capable of entering into numerous chemical reactions.

6. The radicals R of the amino acid units are largely turned outwards.

7. In these radicals there are various functional groups.

Найдите во 2 абзаце текста предложение с эмфатической конструкцией. Переведите его на русский язык.

Прочтите текст до конца, и скажите, где, по-Вашему, могут применяться результаты исследования механизма синтеза белков в организме.

TEXT 9. ENZYMES

Enzymes are biocatalysts formed in cells either as simple or as conjugated proteins with a non-amino acid component. Coenzymes often function in the transfer of electrons or of functional groups (hydrogen atom, acetyl, methyl, amino groups, etc.). The coenzymes are generally identical with vitamins which, at least in higher organisms, represent essential components of their food that cannot be synthesized in their organs. The enzymes accelerate biological reactions by decreasing the activation energy of a given reaction without altering its equilibrium. The mechanism of their action consists in the formation of a complex between enzyme and substrate (ES) which undergoes the chemical reaction proper whereupon the enzyme-product complex (EP) is split to the original enzyme and the product. The existence of some enzyme-substrate complexes has now been proved both indirectly (spetrophotometry) and directly (chemical isolation).

The substrate is bound to the enzyme in the active site or centre which may be visualized as a spatial arrangement of certain amino acids of the protein moiety of the enzyme as well as of the prosthetic group or the coenzyme. Amino acids frequently found to play a role in the active site are serine (through its OH group) and histidine (through the nitrogen of its imidazole). Some enzymes occur in the active form directly upon their synthesis while others are synthesized as inactive proenzymes (pepsinogen, trypsinogen) or exist a part of their life-time in an inactive conformation (allosteric enzymes). They must then be activated by a special process to become functional. Enzyme-catalyzed reactions proceed at different rates, depending on:

the amount or activity of the enzyme,

the concentration of substrate,

the pH and composition of the solution,

temperature,

the presence of activators and inhibitors.

Since the enzyme concentrations in living cells are difficult to estimate we often speak about their activities. Enzyme activity is measured in international units (U), corresponding to an activity converting 1 mmol substrate per min, or, more recently, in katals (kat), corresponding to an activity converting 1 mol substrate per s. Specific activity of an isolated enzyme is expressed in units per mg protein. Specificity of an enzyme defines the range of structure types, the enzyme can attack. If the specificity is absolute the enzyme can catalyze the chemical reaction of a single compound, if it is relative, several related compounds can serve as substrate. Stereospecificity indicates that the enzyme accepts only a certain stereo-isomer (L- or D-form). A given substrate can be convened by one of several theoretically possible reactions catalyzed by different enzymes. This is important in some metabolic control processes.

At low substrate concentrations the enzyme reaction follows first-order kinetics i.e. the rate is proportional to substrate concentration. At very high concentrations of substrate the reaction is of zero order, i.e. the enzyme is saturated by its substrate. Every enzyme reaction has its characteristic Michaelis constant, Kм, which defines the concentration at which the enzyme reaction proceeds at half its maximum rate. It is not identical with the dissociation constant of the ES complex just as its reciprocal is not identical with the association constant of this complex. Every enzyme-catalyzed reaction has its pH optimum.

Notes on the text:

1) conjugated protein – связанный протеин

2) to split – расщеплять

3) to bind (bound) – связывать

4) spatial arrangement – пространственное расположение.

5) to convene – собирать(ся)

Задания к тексту:

5. Прочтите 1 абзац текста и ответьте на следующие вопросы:

1. What is the function of the enzymes?

2. What does the mechanism of their action consist in?

6. Прочтите 2 абзац текста и ответьте на вопрос: “What does the rate of enzyme – catalysed reactions depend on?”

7. Прочтите 3 абзац текста и ответьте на вопрос “What is enzyme activity measured in?”

8. Прочтите 4 абзац текста и скажите, в связи с чем употреблен термин “ first- order kinetics”.

9. Ответьте на вопрос: “What is meant by zero order?”