You are an amino acid in the lumen of the small intestine of a newborn mammal.
You are looking at intestinal epithelial cells that bring important maternal proteins
(immunoglobulins) across their apical surfaces by receptor-mediated endocytosis.
Your ambition is to be part of a receptor that does that job.
You will be asked to describe your adventures during one of three phases of your
A. beginning from the challenge of entering the cell, until you have been loaded
onto an appropriate tRNA. Pg (352-355)
How amino acids enter
- In the small intestine, epithelial cells exist. However the small intestine is covered
with villi which are responsible for absorption of nutrients. Amino acids in particular
are absorbed by active transport. In a way, this is a challenge for the cell because
energy must be used.
- Draw diagram of sodium potassium pump
- There are two main methods of active transport. One method involves the
hydrolysis of ATP into ADP + P. This reaction releases energy which is used to allow
ions to move against the concentration gradient. The other method uses
pumps/active transporters. There are many pumps but the first one that was
discovery was the sodium-potassium pump. This pump works by using ATP to
transfer three molecules of Na+ outside the cell and two molecules of K+ into the
cell. As a result, a concentration gradient is created. You have high [Na+] on outside
and low [Na+] on inside. This concentration gradient is key because we know Na+
wants to go move inside the cell again. But other molecules, such as amino acid, can
be co-transported with Na+ into the cell. More specifically this is called symport (Na+
being cotransported with amino acids).
Translation and loading into tRNA
- a tRNA molecule consists of a single strand of RNA. tRNA has a 3d structure because
of the hydrogen bonds that can form between the RNA strand. There are two main parts of tRNA. An anticodon, a triplet of nucleotides that can form base pairs with a
complementary codon. However, the other part we’re interested in here is the
amino acid attachment site.
Loading of amino acid into tRNA
- Amino acids can covalently link to tRNA at these sites. This is done through the help
of enzymes called aminoacyl tRNA synthetase. There are 20 different types of these
enzymes, 1 to help bind each type of amino acid. The way it is by allowing both the
amino acid and ATP binding on the active site. ATP loses two phosphate groups and
becomes AMP. This gives the amino acid high potential energy. This allows the
appropitae tRNA to come in and bind to the amino acid, AMP is released. The
product is then released from aminoacyl tRNA and is ready for translation. The
product is called aminoacyl tRNA.
B. beginning from initiation of translation, until your incorporation into a folded
The purpose of translation is to create a polypeptide chain. Translation can be split
into three main steps. Initiation, elongation and termination.
Translation occurs in the ribosome (draw a ribosome as seen on pg 356)
Ribosomes help with the coupling of tRNA anticodons with codons on the mRNA
strand. This process begins when a specific region of mRNA binds to a secquence of
RNA on the ribosome. The specific region of mRNA is called the Shine-Dalgarno
sequence/ribosomal binding site. Proteins called initiation factors help with the
binding of these two sequences.
Once binded, an aminoacyl tRNA with a methionine binds to the AUG start codon at
the P site.
This marks the beginning of the second stage which is elongation. A and E sites are
empty, and an mRNA codon is exposed at this site. A corresponding aminoacyl tRNA
binds to the A site. Once the situation occurs where the A and P sites are filled, a peptide bond between the respective amino acids can form. Protein synthesis is
catalyzed by a ribozyme. Once a peptide bond forms, the polypept