Wow! It's amazing how many different ways they can line us up to form a protein. Isn't it strange that just by switching places in line with the amino acid before or behind me, I can alter the entirety of the protein? I've also noticed that once in a while the amino acids themselves seem to be slightly modified. When I asked some of the older amino acids why that was true; they said it took thousands of years of evolution to alter an amino acid in the slightest degree. We were lined up once again and we were processed through the ribosome to become a human protein. Eventually, I became a part of actin, which along with myosin makes up a human muscle. By sliding over each other, the muscle is activated and can manipulate minute changes in position. Right now we are located in the deltoid muscle which means we are moved quite often!
Protein synthesis is simply the "making of proteins." Although the term itself is easy to understand, the multiple steps that a cell in a plant or animal must go through are not. In order to make even one protein, the body must invoke the aid of messenger RNA, transfer RNA, DNA, amino acids, ribosomes, and multiple enzymes. So that this process does not become too overwhelming, let's first go over some of the basics.
A protein is simply a long chain of amino acids linked together by bonds. There are only twenty amino acids consisting of carbon, hydrogen, nitrogen, oxygen, and two that contain sulfur. Ten of these amino acids have side groups that are attracted to water, while the other ten do not. Therefore, when a protein is in a water-based environment, the hydrophobic amino acids fold inwards while the hydrophilic remain on the outside. (This process is aided by chaperone proteins.) The backbone of amino acids form strong covalent bonds and the actual amino acids form temporary weak bonds. These weak bonds allow the amino acids to change shape, remain mobile, and attain flexibility. The most important quality to understand about proteins is that the position of their amino acids determine their function.
Now that we know a little about the basic structure and function of a protein, it will be easier to understand how a protein is made. The three basic steps of protein synthesis are:
1. Transcription: This is where the sequence of
nucleotides present in the double helix DNA form are transcribed
into a single strand of messenger RNA. So that you will understand
the vast amount of work that is present in this step, let's look
at some numbers. Every three nucleotides will become the instructions
for just one amino acid. There are 1,200 nucleotides in just
one gene in a strand of DNA. This means that 400 amino acids
would have to be produced to represent one gene in one strand
of DNA. Considering the huge number of genes in the body, it
is easy to appreciate just how many nucleotide combinations there
are that will eventually encode amino acids.
2. Transferal: In this stage, an amino acid activating enzyme attaches an amino acid to one end of transfer RNA, also called an adaptor molecule. On the other end of the adaptor is a specific three-nucleotide code which will be used when the adaptor reaches the mRNA. Since there are twenty different kinds of amino acids, there are also twenty different kinds of adaptors and amino acid activating enzymes. To simply say that an adaptor and an amino acid are connected is an oversimplification of the process. Let's look closer, step by step.
An ATP molecule docks on the activating enzyme in a place specially prepared for it (think of this space as a reserved parking place).
An amino acid then parks in the next space which was also prepared especially for it.
The ATP molecule and the amino acid drift closer together until they bond, thereby releasing two phosphates. At this point, the amino acid is energized.
The adaptor then comes and parks in the space next to the amino acid (as you may have guessed, the place was also reserved especially for the adaptor).
The adaptor comes closer to the amino acid until they bond.
The activating enzyme finally releases the adaptor with the amino acid attached to one end.
3. Translation: At this point, one amino acid is attached to an adaptor. Remember that it takes many amino acids to make up one protein. Therefore, there must be a way to link these amino acids into a single protein in order to complete protein synthesis. This is where the ribosome comes in which is so good at producing proteins that is often termed a "protein factory." The ribosome "reads" the nucleotide code and associates it with the proper amino acid. Again, let's look at the mechanisms of this system closer.
A messenger RNA attaches to the smaller subunit of the ribosome
The first adaptor comes and matches the first three nucleotides of the messenger.
A second adaptor enters a second dock to the left of the first adaptor.
The backbone links of the two amino acids present at the ends of the adaptors link.
The messenger moves right causing the first adaptor to drop off, leaving an empty space to the left of the second adaptor. It is important to understand that this adaptor has two amino acids at its end.
The process repeats itself as new adaptors arrive, link their amino acid chains, and then push off the previous adaptor. This causes the adaptor's amino acids to grow.
The last triplet on the messenger RNA will not be able to find an adaptor that fits it; this signals STOP.
The ribosome releases the mRNA and the amino acid chain has grown to a protein.
In order to facilitate this process, many ribosomes work at once to produce proteins.
Therefore, protein synthesis is a fairly complex process where amino acids are connected together to form a protein. The distinguishing aspect of this process is that each different order of amino acids causes a different protein to be produced. In this case, serine was one of the amino acids that was used in order to produce a protein in a human.
