Proteins

As we have already seen, the distinguishing characteristic of a given biopolymer is the chemical composition of its elemental (monomer) units. This can be as simple as the CH₂ (i.e. H—C—H) unit present in the chains in lipids to the more complicated amino-acid residue units present in proteins. The latter will provide us with our first example of these vital threads. An amino acid consists of three parts: an electrochemically basic amino group, NH₂, at one of its extremities; an acidic carboxyl group, COOH, at the other end; and, lying between these two ends, a single carbon atom, to which are attached a hydrogen atom and also a side group, usually denoted by the letter R. That centrally-located carbon atom is referred to as the alpha-carbon (α-carbon), to distinguish it from the carboxyl carbon, and the great variety of possible protein structures stems from the fact that the side-group R can have any of approximately 20 different chemical compositions.

The different side groups interact with the surrounding environment in different ways, so the physical properties of a given protein are determined by the sequence of its Rs.

There are two ways in which monomers can become joined together so as to produce a link in the polymer chain, namely either by what is known as addition or by the process called condensation. The former merely involves rearrangement of some of the interatomic bonds, so as produce the link.

Such a process is conservative in that there are no by-products of the polymerization process. The latter, by way of contrast, depends upon the prerequisite removal of certain atoms from one or both of the consolidating units, so this process produces not only the link but also a by-product composed of those surplus atoms.

Proteins are produced by such a condensation process. There is no redistribution of bonds within the molecule. The rearrangements exclusively occur at the termini and each step of the consolidation produces a water molecule as a byproduct. We see, therefore, that the monomers present in proteins are amino-acid molecules from which have been removed one oxygen atom and two hydrogen atoms; these monomers are therefore amino-acid residues, while the by-product is water. The word protein was coined by Jöns Berzelius, it’s stemming from the Greek term for of the first rank.

Define the words

Vital, to denote, to be referred to as, surplus, conservative, a byproduct, to coin.

Complete the sentences

1. As we have already seen, the distinguishing characteristic of a given biopolymer…

2. An amino acid consists of...

3. That centrally-located carbon atom is referred to as…

4. There are two ways in which monomers can become joined together…

5. The polymerization process, by way of contrast,...

6. As can be seen, the monomers present in proteins…

7. The rearrangements of bonds exclusively occur...

Put the following words and word combinations into the gaps

distinguish / prerequisite / thread / residues / stems from / is referred to as / byproduct / vital / denotes

Ionic interactions between oppositely charged amino acid ______________, such as glutamic acid (–) and lysine (+), also hold enzymes together.

Bacteria play a ______________ role both in productivity and in cycling the substances essential to all other life-forms.

An individual mammalian hair is a long, protein-rich filament that extends like a stiff ______________ from a bulblike foundation beneath the skin known as a hair follicle.

The blood type ______________ the class of antigens found on the red blood cell surface.

The resulting tightly packed mass of about 32 cells is called a morula, and each individual cell in the morula ______________ a blastomere.

It is important to ______________ denaturation from dissociation.

Two sets of risks need to be considered. The first ______________ eating genetically modified foods, the other concerns potential ecological effects.

Thus, although haplodiploidy may have facilitated the evolution of eusociality, it is not a necessary ______________.

Reproductive isolating mechanisms can evolve either through random changes or as an incidental ______________ of adaptive evolution.

Match the words in the left column with suitable words in the right column to make up collocations. Translate them into Russian.

distinguishing	bonds
chemical	characteristic
surrounding	composition
interatomic	environment
consolidating	units

Translate the text into Russian orally

Самоорганизация белка. Определенный характер распределения гидрофобных групп в развернутой цепи способствует образованию в ней α- или β-структурных участков в основном за счет локальных взаимодействий внутри данного участка. Одновременно за счет дальних взаимодействий структурного сегмента с другими участками белковой цепи обеспечиваются ее стабилизация и встраивание в белковую глобулу. Это приводит к тому, что образованные в развернутой цепи наиболее вероятные структурные сегменты вторичной структуры предопределяют и структуру целой компактной глобулы, возникающую при сворачивании первичной последовательности. Вместе с тем для α-спиральных белков α-спиральная структура уже доминирует в развернутой цепи, а β-структура доминирует в развернутой цепи β-структурных белков.

Так, для кальций-связывающего белка мышц карпа с доминирующей α-структурой в сформированной глобуле локальные максимумы вероятности образования спиральных участков в развернутой последовательности соответствуют гидрофобным участкам цепи со «спиральным» распределением гидрофобных, инициирующих и терминальных полярных групп. Уже на первых этапах самоорганизации в этом белке вторичной структуры α-спирали подавляют β-структуру практически по всей длине цепи. Аналогичная картина наблюдается и у β-структурных белков (рибонуклеаза А), где β-структура доминирует с самого начала самоорганизации. Конечная структура белковой глобулы является результатом направленного процесса сворачивания цепи с предсуществующими структурными сегментами.

Описаны различные, пока еще далекие от завершенности, молекулярные модели образования третичной структуры белка, в которых учитываются стереохимические особенности α- и β-структур. Например, α-спирали должны упаковываться так, чтобы боковые группы одной спирали («хребты» в положениях 1 — 5 и 1 — 4) входили в промежутки («ущелья» в положении 1-2) между боковыми группами другой спирали. Структурные сегменты соединены, как правило, гидрофобными участками цепи, где отсутствуют гидрофобные связи между сегментами. Именно в «слабых» точках комплекс может «переломиться» с образованием компактной структуры, состоящей уже из коротких нативных структурных сегментов.