AMINO ACID
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1
What are amino acids?
Structural units or monomers of proteins made up of an amino group and a carboxyl group
2
What is the general structure of an amino acid?
Central alpha carbon attached to amino group, carboxyl group, hydrogen and R group
3
What is R group in glycine?
Single H atom
4
What is R group in alanine?
Methyl group
5
What is R group in glutamic acid?
Carboxymethyl / propanoic acid chain
6
What is R group in histidine?
Imidazole ring
7
Which amino acid is achiral?
Glycine
8
Why does glycine provide flexibility?
No side chain, small size, achiral
9
What is significance of alanine?
Hydrophobic interactions and stabilizes protein structure
10
What is significance of glutamic acid?
Acidic amino acid, negatively charged at physiological pH, forms salt bridges, strong buffer
11
What is significance of histidine?
Weakly basic amino acid, involved in enzyme active sites, excellent buffer near physiological pH with pKa ≈ 6
12
What are D and L forms of amino acids?
Two stereoisomeric forms that are mirror images and not superimposable
13
What are D and L forms called?
Enantiomers
14
When is amino acid D-form?
If NH2 group is on right side of alpha carbon
15
When is amino acid L-form?
If NH2 group is on left side of alpha carbon
16
What are standard amino acids?
20 amino acids encoded directly by genetic code and incorporated during ribosomal translation
17
What are non-standard amino acids?
Not encoded by standard genetic code, arise as free metabolic intermediates, not incorporated into proteins
18
Give examples of non-standard amino acids?
Ornithine, citrulline, GABA, homocysteine, taurine, homoserine
19
What are post-translationally modified amino acids?
Standard amino acids chemically modified after protein synthesis
20
Which amino acids are hydroxylated in collagen?
Proline and lysine to 4-hydroxyproline and 4-hydroxylysine
21
Where is gamma carboxy glutamate found?
Clotting factors II, VII, IX and X
22
Where is methyl lysine found?
In histones
23
How are amino acids classified?
Based on nature of side chain and based on metabolic fate
24
What are polar amino acids?
Hydrophilic in nature e.g. glycine, serine, cysteine, threonine, glutamine, lysine, arginine, glutamic acid
25
What are non-polar amino acids?
Hydrophobic in nature e.g. alanine, phenylalanine, tryptophan, valine, leucine, isoleucine, methionine
26
What are glucogenic amino acids?
Catabolism yields pyruvate or TCA cycle intermediate for net synthesis of glucose
27
Examples of glucogenic amino acids?
Alanine, aspartate, glycine, methionine
28
What are ketogenic amino acids?
Catabolism yields acetyl CoA or acetoacetate
29
Which amino acids are purely ketogenic?
Leucine and Lysine
30
Which amino acids are both glucogenic and ketogenic?
Isoleucine, tyrosine, phenylalanine, tryptophan
31
What are ionizable groups in amino acid?
Amino group (-NH2/-NH3+) and carboxyl group (-COOH/-COO-)
32
How does amino group act?
As base, can accept H+
33
How does carboxyl group act?
As acid, can donate H+
34
What is zwitterion?
Molecule that carries both positive and negative charge simultaneously with overall net charge zero, from German zwitter = hybrid
35
What is isoelectric point pI?
Specific pH at which amino acid carries zero net charge and does not migrate in electric field
36
What is net charge at pI?
Zero
37
What is form at pI?
Zwitterion (-NH3+ and -COO-)
38
What is mobility at pI?
No migration in electric field
39
What is solubility at pI?
Minimum solubility
40
What is buffering capacity at pI?
Minimum buffering capacity
41
What is electrical conductivity at pI?
Low conductivity
42
What happens to proteins at pI?
Increased tendency for aggregation / precipitation
43
Why can histidine act as buffer at physiological pH?
Contains imidazole group whose nitrogens can accept or donate H+, pKa ≈ 6.0-6.5 close to physiological pH 7.4
44
What does histidine do when pH < pKa?
Accepts H+ and acts as base
45
What does histidine do when pH > pKa?
Donates H+ and acts as acid
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