B3b: Proteins and mutations

Proteins

What are proteins? Proteins are important chemicals found in your body. There are many different types all with a different job to do. Proteins are polymers (relatively large molecules made from many smaller molecules). Each protein molecule is built up from amino acids. Each protein has its own number and sequence of amino acids. This gives each protein molecule a particular shape, allowing it to carry out a particular function. Different types of protein have different functions. The table shows four examples.

Switch on or off?Different types of cells have different functions because they make different proteins.They only make certain proteins because only some of the full set of genes are used. Some genes are 'switched off' - meaning the proteins they code for cannot be produced.So the 'switched on' genes are the ones that determine the function of the cell.eg. the genes inside muscle cells only code for muscle cells proteins as they are switched on.

Mutations

What are mutations?A mutation is a change in the DNA base sequence.If a mutation occurs within a gene, it could stop the production of the protein the gene normally codes for - or a different protein is produced instead.

What causes mutations?Mutations can be spontaneous (they just happen) - when a chromosome doesn't copy itself correctly.However the chance of mutation increases when exposed to: Ionising Radiation (including X-rays, radioactive substances and ultraviolet light) - the greater the dose of radiation, the greater the chance of mutation. Chemicals (mutagens), such as tar (carcinogens) from cigarette smoke - increase the chance of mutations that could cause cancer.

Harmful Mutations Producing the wrong protein or no protein can be disastrous - especially if that protein is an important enzyme. If a mutation occurs in reproductive cells, then the offspring may develop abnormally or die at an early stage of their development. If a mutation occurs in body cells, the mutant cells can sometimes start to multiply in an uncontrolled way and invade other parts of the body. This is cancer. Haemophilia is an inherited disorder that stops blood from clotting properly - it is caused by a mutated gene. Helpful Mutations Occasionally, a different protein produced after a mutation can actually benefit the organism - the new protein is an improvement on the one it was supposed to be. This gives the organism a survival advantage over the rest of the population. It passes the mutated DNA to its offspring, and they survive better too - soon the mutation becomes common in the population. This is how natural selection and evolution works. 'No Effect' Mutations Some mutations aren't harmful or helpful - they don't change to protein being coded for so they have no effect on the organism. For example, the protein that a mutated gene produces may work just as well as the protein from the non-mutated gene.

Enzymes

Enzymes are biological catalysts – protein molecules that speed up chemical reactions. They catalyse chemical reactions in living cells such. Examples of these chemical reactions are: Respiration Photosynthesis Protein synthesis Active sites The shape of an enzyme determines how it works. Enzymes have active sites that substrate molecules (the substances involved in the chemical reaction) fit into when a reaction happens.The active site has to be the right shape for the substrate molecules to fit into. This means that enzymes have a high specificity for their substrate – a particular type of enzyme will only work with one or a smaller number of substrates. The animation shows how this works: In this animation an enzyme is shown joining two smaller substrate molecules together to make a larger molecule.The mechanism involved is called the 'lock and key' mechanism. Just as a lock will only accept one key, an enzyme will only accept one substrate.

Changing the shape of the active site of an enzyme will cause its reaction to slow down until the shape has changed so much, the substrate no longer fits. When this happens the reaction stops. At this point we say the enzyme is denatured. Denatured enzymes Denaturing is an irreversible change – the enzyme is permanently damaged. Its function is inhibited and the enzyme no longer works. Denaturing can happen at high temperatures and at extreme pH values.Enzymes work slowly at low temperatures too. However, this is because the substrate molecules have less energy and move into the active site more slowly. This is not a permanent change. Q10 Q10 or temperature coefficient is a measure of the rate of change of a reaction when the temperature is increased by 10 °C. Many enzymes have a Q10 of about 2. This means that the rate of reaction doubles when the temperature is increased by 10 °C.Q10 is calculated using this formula:Q10 = rate at higher temperature ÷ rate at lower temperatureFor example, if rate of a certain reaction is 10 units/min at 20 °C and 24 units/min at 30 °C. What is its Q10?Q10 = rate at higher temperature ÷ rate at lower temperature = 24 ÷ 10 = 2.4

Enzyme-catalysed reactions

Enzyme-catalysed reactions Enzymes work best at particular temperatures and pH values. Enzymes and temperature At low temperatures, enzyme reactions are slow. They speed up as the temperature rises until an optimum temperature is reached. After this point the reaction will slow down and eventually stop. The graph shows what happens to enzyme activity when the temperature changes. In the example above, enzyme activity increases steadily between 0 ºC and 40 ºC. It peaks at 40 ºC (the enzyme's optimum temperature) then decreases rapidly. Enzymes and pH Different enzymes work best at different pH values, their optimum pH. Many enzymes work fastest in neutral conditions. Making the solution more acidic or alkaline will slow the reaction down. At extremes of pH the reaction will stop altogether. Some enzymes, such as those used in digestion, are adapted to work faster in unusual pH conditions. For example, stomach enzymes have an optimum pH of 2, which is very acidic. The graph shows what happens to enzyme activity when the pH changes. In the example above, enzyme activity increases between pH 4.5 and pH 8. It peaks at pH 8, then decreases.