Amino Acids and pH Flashcards
Why do we need amino acids?
- Generate metabolic energy
- Substrates for protein synthesis
- Substrates for production of coenzymes
Describe the structure of amino acids.
- Central carbon is an alpha carbon
- R group determines properties
- Adjacent amino acids form peptide (CO-NH) bonds
What is special about all amino acids, apart from glycine?
- ASYMMETRIC
- Exist as enantiomers
Where do the L- and D-enantiomers of amino acids occur?
- L-FORM in all amino acids in humans
- D-FORM in antibiotics/bacterial cell walls
Describe the charges of amino acids at pH 7.4
- Amino group is positively charged (NH3 +)
- Carboxyl group is negatively charged (COO-)
- Amino acid is a zwitterion
When is an amino acid positively/negatively charged/no net charge?
- POSITIVELY - when pH is < pKa of carboxyl group (i.e 1.8-2.4)
- NEGATIVELY - when pH is > pKa of amino group (i.e 8.8-11)
- No net charge at isoelectric point - number of + and - charges are equal
Amino acids are soluble at pH 7.4. Why?
- Amino groups are protonated
- Charged groups form hydrogen bonds with water molecules
How can amino acids be classified?
- Essential or non-essential
- Nature of side-chain
What makes an amino acid essential/non-essential?
- ESSENTIAL - not synthesised in body so taken in through diet e.g valine, lysine
- NON-ESSENTIAL - synthesised in body so don’t need to be taken in through diet e.g alanine, glycine
Describe nonpolar, aliphatic groups.
- R group is nonpolar and hydrophobic
- Promote hydrophobic interactions within protein structures
- Cannot form hydrogen bonds so don’t dissolve in water
Describe polar, uncharged groups.
- R group is more hydrophilic than those of nonpolar amino acids
- Functional groups form hydrogen bonds with water
- EXAMPLE: OH group in serine/ Amide group in glutamine
Describe aromatic groups.
- Contain nonpolar phenyl groups that participate in hydrophobic interactions
- OH group of tyrosine is hydrophilic so forms hydrogen bonds
Describe cysteine.
- Contain thiol groups that form disulphide bonds through oxidation to form cystine
- Present in blood but not very water soluble
Describe methionine
- Nonpolar with bulky hydrophobic side chains
- No thiol groups so no disulphide bonds
- Involved in metabolism - transfers methyl group to other sulfur containing molecules
GIve examples of charged amino acids.
- Acidic aspartate and glutamate have COO- at physiological pH
- Basic lysine and arginine have NH3+ at physiological pH
Describe the bonding that charged groups can undergo.
- Basic amino acids form ionic bonds with negatively charged amino acids
- Acidic and basic amino acids participate in hydrogen bonding
Describe amino acid metabolism.
- Ketogenic amino acids degraded to acetyl CoA, -used for ketogenesis during starvation e.g lysine, leucine
- Ketogenic and aromatic glucogenic amino acids - degraded to ketone bodies and glucose e.g isoleucine, phenylalanine
- Glucogenic amino acids - degraded to glucose in TCA cycle or glycolysis
What allows amino acids to act as buffers?
- Weakly acidic carboxyl groups and weakly basic amino groups
- Ionisable side chains
What can be calculated from the Henderson-Hasselbach equation?
- Relationship between pH of solution and pKa of acid
- When pH = pKa –> [A-] = [HA]. 50% of weak acid is dissociated
- For calculating ionic forms of acid/base drugs
- For calculating how shifts in Bicarbonate ion concentration [HCO3-] and CO2 in ABGs influence pH
What are buffers?
- Consist of HA and A-
- Resist changes in pH when H+ and OH- added
- When acid added, A- neutralises it to form HA
- When base added, HA neutralises it and forms A-
- Maximum buffering within +/- 1 of pKa
Describe the acetic acid curve.
- pKa of 4.8 so buffering region from 3.8-5.8
As pH increases to pK1 and pK2, what happens?
- pK1 - COOH dissociates to form COO-
- pK2 - NH3+ deprotonates to form NH2
How is isoelectric point calculated?
(pK1+pK2)/2
How does gel electrophoresis work?
- Separation of proteins done at a pH above the pI of the major proteins involved
- Net negative charge determines movement towards positively charged cathode
How does pH affect movement of amino acids during electrophoresis?
- POSITIVELY CHARGED (pH<pI) moves towards anode
- NEGATIVELY CHARGED (ph> pI) moves towards cathode
- NEUTRAL - no net charge - doesn’t migrate
How can pI be calculate for amino acids with basic/acidic side chains?
- Between 2 higher pKas for basic
- Between 2 lower pKas for acidic
Why does the body need to regulate pH and what pH is the blood kept at? Give examples of some physiological buffering systems.
- Physiological processes are pH dependent
- Blood pH kept at 7.35-7.45
- BICARBONATE BUFFER SYSTEM/PHOSPHATE BUFFER SYSTEM
Describe the bicarbonate buffer system.
CO2 + H2O ⇄ H2CO3 ⇄ H+ + HCO3-
- pKa of H2CO3 is 6.1
- At low pH, HCO3- absorbs H+ and converts to H2CO3 then CO2.
- At high pH (low H+), H2CO3 dissociate to H+ and HCO3
