Some Familiar Proteins

The hair on your head is an example of an almost pure protein. So is silk. The protein of hair is called keratin by chemists, and the protein of silk is called fibroin.

Both keratin and fibroin are comparatively simple proteins. Their molecules consist of amino acids strung together in more or less a long straight line. Such lines of amino acids are called polypeptides.

In the 1940’s chemists learned to manufacture quite long polypeptide chains in the laboratory. They used only one or two different amino acids in doing so, however.

Then, in the 1950’s, chemists learned how to put together amino acids of many different varieties, one by one, just in a particular order. By 1960, a protein built up of 23 amino acids, was manufactured in the laboratory. It was found to behave just like a similar small mole­cule formed by the body. However, 23 amino acids are a long way from the hundreds and thousands of amino acids found in the larger proteins made by the body.

Still, fibroin isn’t much more complex than these laboratory crea­tions. Its molecule contrasts of over 250 amino acids of 14 different kinds. Eighty-five per cent of the molecule is made up of only three different amino acids, and those three happen to be the simplest of all. It is for this reason¹ that silk doesn't play a vital role in life. It is just used by the silkworm² to make a soft cocoon for itself.

Protein such as fibroin and keratin are called fibrous proteins. In general, fibrous proteins are strong, sturdy (крепкий) and tough (прочный). Keratin, for instance, is the chief protein not only of hair, but of skin, nails (ногти), hooves (копыта), scales (чешуя), horns (pora) and feathers (перья). Another important fibrous protein is collagen, which occurs in cartilage (xpящ), ligaments (связки) and tendons (cyxoжилия).

The really important proteins, however, are the globular proteins. In these, the polypeptide chains are not merely straight lines, but existing complicated loops (петля) and twists (изгиб) which are never quite the same in any two different proteins.

Notes

1. it is for this reason – именно по этой причине

2. silkworm – шелковичный червь

Enzymes and Genes

The nucleus of the cell is in charge of cell division. Unfortunately, most of the details of the process are as yet unknown. Still we can describe some of them.

Inside the nucleus are small patches that can react with certain dyes to become strongly coloured. Biologists noticed them for that reason and called the material in the patches chromatin from the Greek word for colour.

In the process of cell division, the chromatin collects into little rods of varying size. The rods are called chromosomes. In the nuclei of human cells are forty-six such chromosomes, existing in pairs. There are twenty-three pairs of chromosomes, in other words. Each kind of creature has its own fixed number of chromosomes. A rat (крыса) has thirty-eight chromosomes, a grasshopper (кузнечик) twenty-four and a housefly (муха) only twelve. A crayfish (рак) on the other hand, has over two hundred chromosomes.

Before a cell divides, every chromosome lines up in the centre of the cell and splits in two. The two halves of each chromosome move apart and when the cell divides, each new cell has a duplicate of all the original chromosomes.

It is these chromosomes that control a cell’s characteristics. A cell’s nature is determined by the kind of chromosomes it has. Every chromosome is actually a chain of protein molecules which are called genes. Genes are strung along a chromosome as beads are in in a necklace. The genes have a certain chemical resemblance to viruses.

Each gene is thought to control a single characteristic of an organism. For instance, there is a gene for blue eyes and one for brown eyes; one for straight hair and one for wavy hair. Every human being has thousands of different genes scattered through¹ his various chromosomes. When­ever a chromosome splits in two, during cell division, each gene dupli­cates itself exactly and both daughter cells get one apiece.How does a gene control a particular characteristic? Many people now think that each gene is in charge of manufacturing one particular enzyme in the cell.

But how does a gene manufacture an enzyme? For that matter, how does a gene duplicate itself? This is probably the most important un­answered question in biochemistry today. There are theories, of course. There are enzymes that take proteins apart and separate them into amino acids. These protein splitters can also put amino acids back together again.

Apparently, then, the beef (мясо) protein we eat or milk protein, or wheat (пшеница) protein is separated into amino acids and then put together in a different arrangement to make human protein. But how is the arrangement figured out², when there are so many possibi­lities?

Here is where the gene comes in³. Genes are nucleo-proteins. The non-protein part of the molecule is the nucleic acid. Each gene contains its own variety of nucleic acid. Each different nucleic acid some­how acts as a model for the formation of a particular enzyme. Nucleic acids, therefore, control amino acid arrangements.

How? Chemists just began working out the method in the 1950’s. The nucleic acid of the chromosomes forms a "messenger"⁴ molecule which leaves the nucleus and joins particles in the cytoplasm which are called ribosomes.

In the ribosomes are tiny fragments of nucleic acid molecules. There are a number of kinds of these fragments and each will attach its own particular type of amino acid. These nucleic acid fragments carry their amino acids to the "messenger" molecule and use its structure as a guide. They line up to match the structure and each transfers its amino acid. In this way, an entire protein molecule is formed with an exact structure according to the original design of the chromosome’s nucleic acid.

You may wonder how enzymes can control characteristics. How can they decide blue or brown eyes, for instance? Well, eyecolour is due to a pigment called melanin. When the eyes contain very little melanin, they appear blue. With more melanin, they are brown. Melanin is formed in the body as a result of a chemical reaction which is catalysed by the enzyme, tyrosinase. The amount of the formed melanin de­pends upon the amount of tyrosinase present. Possession of a gene producing much tyrosinase will result in brown eyes. A gene that produces less tyrosinase makes for blue eyes.

What happens when a cell splits in two without proper duplication of genes? Sometimes the daughter cells just can’t live. At other times, the cells survive, but with a changed chemistry. Some biochemists think that cancer (рак) cells may originate as the result of such imperfect duplications.

Notes

1. scattered through – разбросанный

2. figure out – вычислять

3. Here is where the gene comes in. – И вот к этому ген имеет прямое отноше­ние.

4. messenger – курьер, разносчик

5. due to – обусловлен, благодаря, из-за