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Proteins
4. Protein synthesis and protein engineering Page 16
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Bacteria can help make artificial snow		 4.2 Protein biosynthesis
Viruses, bacteria, plants and humans are all collections of chemicals. The processes that we recognise as characteristic of living things, growth, development, digestion, respiration, reproduction and so on, are all complex series of chemical reactions.

How do organisms control these reactions? What makes one organism different from another and what makes one individual different from all others of the same species? Everything about an organism depends on the proteins it makes; these differ from species to species and from individual to individual. The catalysts that control life’s reactions are proteins known as enzymes (see Chapter 6). Proteins dictate the nature and speed of chemical reactions in an organism and so control what the organism is and does.

In this section we will look at how an organism ¢ knows’ what proteins to make and how it builds them from a-amino acid units. The chemistry is complicated and researchers are still uncovering new details _ we will only give an outline.
diagram of biosynthesis
Figure 2
An outline of protein biosynthesis		 Organisms keep the information they need to build proteins in genetic material stored in their cells. This is made up from long polymer molecules of nucleic acid . Four different monomer units (nucleotides) link to make nucleic acid. The sequence of these nucleotides holds the information for the primary structure of the organism’s proteins. Each a-amino acid is coded for by a particular sequence of just three nucleotides called a triplet. There are more than enough different triplets to code for all twenty a -amino acids. A gene is a section of a nucleic acid molecule that codes for a particular polypeptide .

You can think of the genetic material as a molecular database. Information can be taken from it and used. Information can be copied and passed on to new cells when the cell divides or to the offspring when the organism reproduces.

The way organisms make proteins (protein biosynthesis) shares some similarities with the solid state peptide synthesis described in the previous section. The a-amino acids are brought up one at a time to a growing polypeptide chain held on a support.

Messenger molecules contain copies of the information from the genetic material and carry it to the parts of the cell where the proteins are put together. The proteins are assembled on ribosomes (Figure 2). Transfer molecules bring specific a -amino acids to this assembly point where they covalently link to the growing polypeptide chain. Each transfer molecule recognises a particular triplet code on the messengers so the a-amino acids link in sequence to give the correct primary structure. Molecular recognition plays a vital part in making the process work. You can read more about it in Chapters 6 and 7.
Copying and using the genetic information involves chemical reactions and these too are controlled by enzyme proteins.

We have seen in Chapter 3 that there is more to a protein than its primary structure. How, then, does the protein twist, fold and stick into its secondary and tertiary structure ? In test tube experiments denatured proteins may re-nature without any help, but this is unlikely to happen in a cell as there are all sorts of other molecules that the protein chain could stick to instead of itself. Protein chemists have recently discovered a group of proteins called chaperones. Although the work is at an early stage,

it seems that these chaperones link to different parts of the polypeptide chain and bring them together in an organised way. It looks as though chaperones are particularly good at holding on to oily (non-polar) regions.

Questions:

1. Find out what you can about the structure of nucleic acids.

2. How many different triplet codes can you make from four nucleotides?

3. Some triplet codes do not code for a -amino acids. What might they code for? Think of the other messages that the protein-making machinery would need to receive.

Antifreezes, ice nucleators and snow-making machines

Molecules in liquid water are continually moving around. They do make hydrogen bonds with each other, but at temperatures above 4°C, these are quickly broken again.

At 0°C liquid water and ice exist together in equilibrium; below 0°C the position of equilibrium shifts strongly towards the formation of ice. Here the molecules are held in a regular arrangement by permanent hydrogen bonds. Before ice can form, though, the water molecules have to get over a small energy barrier. Pure water can be cooled to - 40°C without freezing by preventing the molecules from making it over the barrier.

Any material that raises the height of the barrier will discourage ice formation; any material that can lower the barrier will encourage ice formation.

During winter the polar oceans are at the freezing point of seawater (- 1.9°C). The fish that live in these environments survive, though, without the water content of their bodies freezing. They manage this by making antifreeze proteins that stick onto the surface of any small ice crystals that may start to form inside their blood or other body fluids. This stops more water molecules from joining and so prevents further growth of the crystals.

California and Florida have warm climates and experience frosts only occasionally. Crops grown in regions like these are particularly open to damage when frosts do occur. The financial losses to farmers can be enormous - possibly as much as $14 billion per year across the USA.

Formation of ice crystals on the surface of leaves and fruit is partly responsible for the damage. The ice crystals can only start to grow if they have nuclei to stick to. All sorts of small particles can act as the nuclei. A bacterium Pseudomonas syringae, that grows naturally on plants, has a lot to answer for. It produces a protein on its surface that is especially good at attracting water molecules and starting ice crystal growth.

Scientists have produced a new strain of P. syringae by deleting the gene that codes for this protein. The new strain cannot make the ice nucleator. Crops sprayed with the modified organism have shown resistance to frost damage at temperatures as low as - 10°C.

Microbiologists have developed ways to grow unmodified P. syringae using fermentation technology. They freeze-dry and sterilise the product to give a material that contains high levels of the icemaking protein. The product is sold for use in snow-making machines where water, treated with the protein, is sprayed on to ski slopes at temperatures below 0°C. The protein speeds up the formation of ice crystals so that they form as snow flakes before the water droplets hit the ground.
snow slopes photo
Bacteria can help make artificial snow.
Unilever Education Advanced Series: Proteins
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