Proteins And The Body by Jane Thurnell-Read
Although cells synthesise many different chemicals, a large part of the cellular machinery is devoted to producing proteins. Proteins determine the physical and chemical characteristics of cells and so are vitally important.
Proteins are used to produce enzymes, some hormones, organelles (cellular machinery), muscle (e.g. actin and myosin), structural components of skin and hair (e.g. collagen and keratin), plasma (blood) protein, antibodies, etc. The instructions for producing proteins are coded within the genes.
Proteins consist of amino acids that always contain carbon, hydrogen, oxygen and nitrogen. Proteins are giant molecules made by linking large numbers of amino acids, end to end, so they form a chain. In nature more than 100 amino acids are found, but only 20 are used in humans. The reason for this is that these 20 provide all the chemical and size groups needed to make a very large number of proteins. These 20 different amino acids join up in a variety of ways to make approximately 250,000 different human proteins. The same amino acid can occur many times along a chain making up a specific protein.
The amino acids are like letters and the proteins are like very big words. As with words the exact position of the letters is important: ‘male’ and ‘lame’ produce completely different images even though they are made up of exactly the same letters. The amino acids are made up of other letters - the DNA bases (nucleotides):
There are only 4 DNA bases but there are 20 amino acids, so a sequence of three bases (a triplet) is used to code for each amino acid, e.g.
C-C-G = glycine
A-G-T = serine
If there is the sequence C-C-G-A-G-T this signals the amino glycine followed by the amino serine in the protein ‘recipe’. An insulin molecule consists of 51 amino acids; haemoglobin is 600 amino acids and collagen is 3000.
When all the amino acids are in place, they fold and coil and takes on the characteristic shape for that protein. The shape is determined by the function of the protein, e.g. keratin is in flat, flexible sheets suitable for inclusion in skin; some muscle proteins are long and thin; haemoglobin has spaces for haem to fit in so that it can be carried around the body.
Proteins are synthesised in the cytoplasm (between the nucleus and the outer cell membrane), but the instruction for how to do this is located within the nucleus of the cell, so the cell has to have a mechanism for getting the instructions to the production site.
The process can be summarised as:
DNA to RNA to amino acids to protein
or to put it another way the process involves:
transcribing/copying then translating then building
Here's the detail of this process :
1. A copy of the blueprint is made within the nucleus.
This process is called transcription, as one strand of the DNA (deoxyribonucleic acid) instructions is rewritten in RNA (ribonucleic acid) language by being encoded in a strand of RNA called messenger RNA (mRNA). Approximately 1% of the DNA (exons) is copied like this. The non-coding DNA (introns) is ignored. The RNA bases are used for this, and each sequence of three denotes a particular amino acid. (In the RNA bases uracil replaces thymine). This is called a codon. It is done in this way because the DNA molecules are too big to leave the nucleus, but mRNA is able to leave the nucleus through small pores and move into the cytoplasm. This may seem an unnecessarily long-winded process, but the original instructions are in DNA because DNA is more stable over time than RNA.
2. In the cytoplasm it combines with a ribosome, which is where new proteins are assembled.
Ribosomes are tiny machines that move along the mRNA, translating its message into amino acids. Ribosomes can be unattached (free ribosomes) or attached to the endoplasmic reticulum (membrane-bound ribosomes). The unattached ones are generally producing proteins for use within the cell, and the attached ones are producing proteins for use outside the cells.
3. Transfer ribonucleic acid molecules (tRNA) pick up loose amino acids in the cytoplasm, one per tRNA molecule.
There are different types of tRNA for each amino acid.
4. The tRNA molecule with its amino acid in tow links up with the ribosome.
5. The amino acid is attached at the correct place in the emerging protein chain.
6. When the protein is complete, it breaks off and folds into its particular characteristic shape. The ribosome and mRNA become ready for re-use.
Most protein production runs smoothly (otherwise there would be far more health problems than there are), but things can go wrong:
- Shortage of raw materials: the relevant amino acids may not be present in the food we eat.
- Inability to metabolise amino acids.
- Substances can affect the production of mRNA, e.g. some drugs.
The proteins that are made in this way need to be transferred to where they are needed, but they could become damaged on the way or damage other material, so they are packaged.
Part of the function of the endoplasmic reticulum (ER) is to keep the proteins that have been made separate from the other contents of the cell, so they are sent into the tubes of the ER. In these tubes they may be modified by adding a carbohydrate molecule. This appears to act as a label and determines where the molecule is sent. The molecule ends up in a small swelling on the ER close to the Golgi complex. The molecule is transferred into the Golgi complex and sorted according to its “label”. It may also be changed from an inert to an active form within the Golgi complex (e.g. pro-insulin becomes insulin within the Golgi complex of the pancreatic cells). It is then put into a container for onward transport.
There are two types of containers:
- Transport molecules that could be highly toxic in the wrong situation, e.g. molecules used in antibodies.
- Recycle damaged cell contents by breaking some down for re-use in the cell and transporting away toxins.
- Act as a storage place for strong chemicals that are only used within the cell in certain circumstances.
- Take proteins such as hormones where needed.
- Store concentrated versions of chemicals to minimise space but have them available quickly should the need arise.
The container is released when the cell gets a message that it is wanted. The messages come via the nervous and endocrine system. The container leaves the Golgi complex on the opposite side to which it entered and fuses with the outer cell membrane.
This completes the production of proteins.
Copyright 2013- 2014 Jane Thurnell-Read