DLAproperties Of Amino Acids Flashcards
Where are peptide bonds formed?
Between the a-carboxyl group of one amino acid and the amino group of another
How are peptide bonds formed?
The process of translation
Exception is the synthesis o& the tripeptide glutathione
What are the characteristics of the peptide bond?
- The peptide bond has partial (40%) double-bond character
- Peptide bond is rigid and planar
- the peptide bond is uncharged but polar
- The —C=O and —NH groups of the peptide bond May form hydrogen bonds
- May form repetitive hydrogen bonds
- Stabilizes the a-helical and B-sheet structures
Describe the configuration of the peptide bond
- Trans configuration minimizes steric hindrance
- Side chains are in trans configuration
- Rotation is possible at the a-carbons
- Peptide bonds do not break during denaturation of proteins
- Denaturation involves changing the conformational shape of a protein
Aspartate and glutamate have…
Negative (acidic) side-chains at physiological pH (pk about 4)
What are the precursors to biologically active amines?
The a-amino acids are precursors for many biological active amines
The amino group is retained (amines) and the a-carboxyl group is enzymatically removed as CO2 (decarboxylated)
What is GABA?
A neurotransmitter
What is histamine ?
Mediated allergic and inflammatory responses, gastric secretion and neurotransmission
How is GABA formed?
GABA(y-aminobutyric acid) is formed by decarboxylation of the a-carboxyl group of glutamate
How is histamine formed?
Formed by decarboxylation of histidine by histidine decarboxylase
What are the functions of serotonin ?
- Pain perception
- Affective disorders
- Regulation of sleep, temperature and blood pressure
Explain the transformation of tryptophan to serotonin
- Tryptophan+ tetrahydrobiopterin —> 5-hydroxytryptophan and dihydrobiopterin (+H2O) by hydroxylase
- 5-hydroxy-tryptophan —> serotonin by aromatic amino acid decarboxylase PLP
How does Prozac (fluoxetine) work as an antidepressant?
- Prozac (fluoxetine) as a selective serotonin reuptake inhibitors (SSRI)
- An antidepressant drug that blocks serotonin elimination from synaptic cleft
Enhances the action of the neurotransmitter
What are the Catecholamines?
Dopamine, tyrosine, and epinephrine are formed from tyrosine
Summarize dopamine formation
- Tyrosine is hydroxylated to dihydroxy-phenylalanine (L-DOPA)
- L-DOPA is decarboxylated to dopamine, the first Catecholamine
What are dopamine functions?
Dopamine can act as neurotransmitter or it can be used as an intermediate for the synthesis of norepinephrine and epinephrine
Dopamine and norepinephrine are mainly neurotransmitters in the brain and the autonomic nervous system
Epinephrine and norepinephrine may also act as hormones
What are the functions of epinephrine?
Epinephrine and norepinephrine are released inti the blood from the adrenal medulla during stress situations
Epinephrine (fight or flight) leads to:
-Release of glucose from the liver
- Release of fatty acids from fat cells
- Elevated blood pressure
- Increased heart output
Describe positive and negative charges in amino acids at neutral pH
The a-amino acid carboxyl group is negatively charged at:
-physiological pH
-The pKa for the a-carboxyl groups is about 2
The amino group of the a-amino acids is positively charged
- At physiological pH
- The pKa for the a-amino group is about 9-10
Some amino acids have a side chain that may carry a charge at physiological pH
How is a swifter ion formed?
At physiological pH, a possible charge of the side chain will determine the overall charge of an amino acid
If the side chain doesn’t have a dissociable proton, then a zwitterion is formed at ~neutral pH; the amino acid carries no net charge
What are the positive/basic side chains amino acids?
Sometimes histidine, dependent on exact pH
Arginine (Arg)and lysine (Lys)(pKa for both above 10)
Histidine (his) pKa about 6
Explain the titration curve of alanine
Alanine has two ionizable groups= two pkas
Two pKa gives two buffering regions
Some aa will have 3 pKas due to the ionizable nature of their side chains
- aa capable of acting as either an acid or base - amphoteric
- Most aa are not soluble as physiological buffers as pKas are too high or low I.e. ~2.3 or 8-9
What defines the buffer region of an amino acid?
The pKa defines the buffer region
The pI is the isoelectric point- no net charge
Explain the histidine side chain buffer
-The histidine side chain has an imidiazole ring (pKa 6) which allows buffering of a possible drop in intracellular pH, which is normally about pH 7
- The pKa of histidine May be shifted in proteins to 6-7 due to local chemical environment of the protein
- Histidine would then be even more favorable to buffer the normal pH inside of cells
What are the main blood buffer systems?
- Phosphate buffer
- Hb buffer in RBC (hemoglobin is rich in histidine side chains)
- Bicarbonate buffer in plasma
What are the general structure of amino acids?
An a-carbon atom -assymetric (except in glycine)-linked to:
-A Carboxylic acid group
- An amino group
- An H atom
- A variable R group (side chain): any molecule can replace the ‘R’. For instance glycine simply has an -H instead and alanine a -CH3
- In total there are 22 different R-groups and different three letter abbreviations to represent the amino acid (Asn, Glu)
Explain the stereochemistry of Amino Acids
19 of the common amino acids are chiral (glycine aren’t chiral)
2 non-superimposable mirror images -stereoisomers
- D & L forms based on the glyceraldehyde rule
- Determined by the position of the -amino group, instead of -OH
- Naturally occurring proteins are L-amino acids
- Rare-D-amino acids in some antibiotics & peptidoglycan of bacterial cell wall
How is proline different from amino acids ?
It has secondary amino group and it’s side chain forms a rigid ring structure
How do peptide bonds formed with proline take a specific shape?
- Lead to bends in the protein chain
- Collagen is rich in proline residues
- Interrupt the a-helical structure
- Proline is an a-helix breaker
What amino acids have nonpolar, aliphatic side chains?
Glycine, alanine and proline
What amino acids have nonpolar, aliphatic side chains(branched ) ?
Valine, leucine, and isoleucine
What amino acids have aromatic side chains ?
Phenylalanine, tryptophan, and tyrosine
Contrast Trp and Tyr have UV light absorbance.
UV light absorbance by aromatic amino acids
Absorbance at 280 no is approximately 4x higher for TRP than for TYR
-Phenylalanine doesn’t show much absorbance
What are the sulfur containing methionine and cysteine ?
Methionine and cysteine
Methionine is also grouped under nonpolar side chains
Cysteine is grouped under uncharged polar side chains at neutral pH
Describe cysteine
Contains a sulfhydryl group
2 cysteine residues can be used to form a disulfide bond
2 “free” cysteine May oxidize to form cystine
What are the polar uncharged amino acids:
asparagine and glutamine
Asparagine is a derivative of Aspartate
Glutamine is a derivative of glutamine
What are the polar, uncharged amino acids?
Serine, threonine
Serine & threonine residues contain a hydroxyl group
May be:
- Phosphorylated
- Glycosylation
