Prokaryotic Protein Synthesis

This is where we start getting into the nitty-gritty details. Joy :( I'll try and make this as light as possible, but no guarantees!

Steps in protein synthesis

The steps involved in protein synthesis are activation, initiation, elongation and termination. Activation is the "activating" of a tRNA by joining it to an amino acid. Initiation is the assembly of the ribosomes and first tRNA onto the mRNA. Elongation and termination are pretty self-explanatory. I'll go into them in more detail in a bit.

Be able to describe key functional aspects of key molecules (eg tRNA, mRNA)

This should be ingrained into your very soul by now. I'm not sure which other key functional molecules he's referring to, but... mRNA carries the message, tRNA carries the amino acid residues to the message, yada yada yada.

Molecular components required for each step
Be able to describe the role of each component in protein synthesis
Role of protein factors in each step of protein synthesis (eg initiation factors) – you do need to learn the detailed mechanism.
Energy requirements for protein synthesis

Activation requires four components: tRNA, amino acids, aminoacyl-tRNA synthetases and ATP. Aminoacyl-tRNA synthetases are enzymes that catalyse the linkage between the carboxyl group on the amino acid with the 3' end of the tRNA. This requires ATP. Instead of breaking down ATP into ADP like most reactions, though, ATP is broken down into AMP, releasing pyrophosphate. This is equivalent to requiring two ATP molecules.

Initiation requires five components: tRNA_i^fMet, mRNA, 30S and 50S ribosomal subunits, initiation factors (which are imaginatively named IF-1, IF-2 and IF-3) and GTP. When it's time for initiation to begin, the 30S subunit of the ribosome binds IF-3, IF-1 and then the mRNA. In the next step, IF-2, which is bound to GTP, also binds, recruiting the very first tRNA in the process. This very first tRNA is called tRNA_i^fMet. The "i" means "initial," and "fMet" means "formylated methionine." (The very first methionine in a prokaryotic protein synthesis has a formyl group, which is kinda like an acetaldehyde group, attached to it. Don't ask me why this doesn't happen in eukaryotes, because nobody really knows, least of all me.) In the third and final step of initiation, the 50S subunit binds, GTP is hydrolysed to form GDP, and the initiation factors dissociate.

Elongation also requires five components: ribosomes, aminoacyl-tRNAs, elongation factors, mRNA and GTP. In elongation, an aminoacyl-tRNA (which is basically just an amino acid bound to a tRNA) binds to an elongation factor called EF-Tu, which carries GTP. EF-Tu brings the aminoacyl-tRNA to the A site of the ribosome (you can read more about binding sites in an earlier post of mine). After doing this, the GTP is hydrolysed to form GDP. After that, the peptide bond forms between the amino acid chain at the P site and the amino acid at the A site, and then the ribosome moves along. I've blogged about this before, so I'm not keen to type up a description again right now (unless you guys nag me, of course).

Termination is pretty simple with regards to number of components. Only two components are needed: release factors and GTP. (Yup, all of the steps use GTP for energy, except for activation which uses ATP.) When a stop codon is reached, release factors (imaginatively named RF-1, RF-2 and RF-3) will bind. Release factors hydrolyse the terminal peptide bond, releasing the peptide. The tRNA is also released from the ribosome. The subunits then dissociate.

After the subunits dissociate, they exist in dynamic equilibrium between a full 70S ribosome and separated 30S/50S subunits. However, only the 30S subunits can actually bind to initiation factors and start the process over again.