45 Questions
What is a zwitterion?
A state of an amino acid with a net charge of zero
What happens to the carboxyl group of amino acids at low pH?
It is protonated
What is the charge of amino acids at high pH?
Negative
Where are amino acid transporters/carriers located?
Apical brush border membrane of epithelial cells
What is the function of the Na-H exchange in the luminal membrane?
To establish an electrochemical gradient
What is co-transported with dipeptides and tripeptides in the luminal membrane?
H+
What type of transport is used for most amino acids?
Facilitated transport
What is a type of antibiotic that can be transported by the peptide transporter?
Beta-lactam
What is the function of the peptide transporter in the absorption of antibiotics?
To enhance the absorption of antibiotics
What type of amino acids are released by Carboxypeptidase A?
Hydrophobic amino acids
What is the primary mechanism of amino acid transport in enterocytes?
Creation of Na+ gradient
Where are the amino acids absorbed from in the intestine?
Intestinal lumen
What is the role of Na+/K+ ATPase pump in amino acid transport?
Pumps Na+ out of the cell
What type of transport proteins are involved in amino acid absorption?
Semi-specific Na+-dependent transport proteins
What is the purpose of the secondary transport mechanism in amino acid transport?
To allow the cell to concentrate AAs from the intestinal lumen
What type of amino acids are released by Carboxypeptidase B?
Basic amino acids
Where does the facilitated diffusion of amino acids occur?
Serosal side
What is the location of the Na+/K+ ATPase pump in enterocytes?
Serosal membrane
Which amino acid is classified as aromatic and nonpolar?
Tryptophan
Which of the following amino acids is negatively-charged?
Aspartic Acid
What is the characteristic of basic amino acids?
They have an extra amino group in their R group.
Which amino acid is classified as sulfur-containing and nonpolar?
Methionine
Which of the following amino acids is essential?
Phenylalanine
Which amino acid is classified as hydroxyl-containing and polar?
All of the above
Which of the following amino acids is non-essential?
Asparagine
What is the characteristic of acidic amino acids?
They have an extra carboxyl group in their R group.
Which of the following amino acids is classified as aromatic and polar?
Tyrosine
What is the effect of a defect/mutation in the 6th position of hemoglobin in sickle cell disease?
Results in crystallization of hemoglobin molecules when O2 levels in the blood are low
What is the substitution in sickle cell hemoglobin?
Glutamic acid is substituted with valine
What is a conservative replacement/mutation/substitution?
Replacing an amino acid with a different amino acid of the same biochemical properties
What is the result of the substitution of glutamic acid with valine in sickle cell hemoglobin?
Results in the formation of sickled red blood cells
What is the effect of low oxygen levels in the blood on hemoglobin in sickle cell disease?
Causes hemoglobin to crystallize
Is the substitution of glutamic acid for valine in sickle cell hemoglobin a conservative replacement?
No
What is the result of sickling of red blood cells?
They become stuck in small blood vessels
What is the normal amino acid in the 6th position of hemoglobin?
Glutamic acid
What happens to hemoglobin molecules when oxygen levels in the blood are low in sickle cell disease?
They crystallize
What is the initial charge of the amino acid when the pKa is below 2.01?
+2
What is the charge of the amino acid when the first pKa value is reached?
+1
What is the charge of the amino acid when the second pKa value is reached?
0
What is the charge of the amino acid when the third pKa value is reached?
-1
What is the correct sequence of charges for Arginine?
+2, +1, 0, -1
What is the purpose of the shortcut for the charges of amino acids?
To know what pKa value to use for computing the pI
What is the rule for acidic and basic amino acids?
The pI is located between 2 nearest pKa values
How do you determine the pI of an amino acid?
By taking the average of the 2 nearest pKa values
What is the charge of glutamic acid at pH 3.2?
0
Study Notes
Amino Acid Chemistry
- Amino acids can exist as zwitterions, a state with no net charge, with both positive and negative charges.
- At low pH, the carboxyl group of amino acids is protonated (-COOH), making them positively charged.
- At high pH, the alpha-amino group of amino acids is deprotonated (NH2), making them negatively charged.
Amino Acid Transporters
- Amino acid transporters are located in the apical brush border membrane of epithelial cells.
- These carriers have overlapping specificity for different amino acids.
- There are transporters for:
- Neutral amino acids
- Proline and hydroxyproline
- Acidic amino acids
- Basic amino acids (lysine, arginine, ornithine) and cystine
- Most amino acids are transported by more than one transport system.
Dipeptides and Tripeptides Transport
- Dipeptides and tripeptides are co-transported by the H+ in the luminal membrane.
- An electrochemical gradient is established by Na-H exchange, powered by Na-K ATPase pump.
- The peptide transporter also accepts beta-lactam antibiotics.
Exopeptidases
- Exopeptidases are zinc peptidases that remove amino acids from the carboxyl end of a polypeptide chain.
- Carboxypeptidase A releases hydrophobic amino acids (val, leu, ile, ala).
- Carboxypeptidase B releases basic amino acids (arg, lys).
Protein Digestion and Absorption
- Digested amino acids are absorbed by enterocytes principally by semi-specific Na+-dependent transport proteins.
- The primary transport mechanism is the creation of a Na+ gradient, driven by the pumping of Na+ out of the cell by a Na+, K+-ATPase pump.
- The secondary transport mechanism is the coupling of amino acids to the influx of sodium, allowing the cell to concentrate amino acids from the intestinal lumen.
Amino Acid Classification
- Aromatic amino acids:
- Nonpolar: tryptophan, phenylalanine
- Polar: tyrosine
- Basic amino acids:
- Positively-charged: lysine, arginine, histidine
- Acidic amino acids:
- Negatively-charged: aspartic acid, glutamic acid
- Amino acids containing an amide group:
- Asparagine, glutamine
- Essential and non-essential amino acids
- Essential: phenylalanine, valine, tryptophan, threonine, isoleucine, methionine, lysine, leucine
- Non-essential: alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, tyrosine, serine
Sulfur-Containing Amino Acids
- Polar: cysteine
- Nonpolar: methionine
Hydroxyl-Containing Amino Acids
- Serine, threonine, and tyrosine
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