Option: ChemBio Flashcards
What are the 4 types of amino acid groups?
Hydrophobic
Polar
Acidic
Basic
and “others”
What are the hydrophobic amino acids?
What are the polar group amino acids?
What are the acidic group amino acids?
Aspartic acid (Asp) - R = CH2CO2H
Glutamic acid (Glu) - R = CH2CH2CO2H
What are the basic group amino acids?
Lysine (Lys) - R = CH2CH2CH2CH2NH2
Arginine (Arg) - R = CH2CH2CH2NHC(=NH)NH2
What are the groups which are not standard regular amino acids?
Glycine (R=H) and
What is ribosomal peptide biosynthesis?
Protein synthesis from mRNA using the ribosome
Translation
What occurs during translation which introduces errors?
Different amino acid residues naturally inserted
Example: Selenocysteine (cysteine with Se instead of S)
How can non-ribosomal peptide (NRP) synthesis occurs biologically?
Large multi-subunit enzymes which make peptides which aren’t from a genetic code
Often v large proteins
When are disulfide bonds observed?
Between thiol groups of cysteine residues in the same (intra-chain) or other chain (inter-chain)
Most common in extracellular proteins
What is the difference between cysteine and cystine?
Cysteine = thiol form
Cystine = disulfide form
What is post-translational modifications?
Proteins extensively covalently modified after their synthesis
Done by enzymes
How can amino acids be modified post-translation using phosphtaes?
Phosphorylation/dephosphorylation - serine, threonine, and tyrosine
This is used for signaling pathways
What are glycoproteins and how/when are they made?
Protein with sugars added to OH of amino acids (serine, threonine, or amides of asn and gln)
Done via gycosylation which is post-translation
What is collagen?
Extracellular protein occuring in connective tissues
Initially made as individual chains with post-translation cross-linking
What is the mechanism for cross-linking in collagen?
Why are proteins synthesised in labs?
Confirm structures and analysis
Prepare for biological evaluation
Used as antigens for preparing antibodies
Make peptide-based medicines
What is the primary structure of a peptide?
Amino acid sequence and if there are other cross-links
What are the different approaches for sequencing proteins?
Chemical analysis
Nucleic acid sequencing
X-Ray crystallography
Mass Spec
What chemical analysis is done for amino acid sequencing?
N&C-terminal analysis or degredation then derivatization
Edman degredation - N-terminal modification followed by hydrolysis
How is mass spec used for amino acid sequencing?
Determine total mass
Then sequential fragmentation in spectrometer can be measured
How do you do degredation and derivatization in proteins?
Hydrolysis - leaves all amino acids are H-AA-OH
(AA = amino acid)
Then Derivatised by chromogenic / fluorescence label and identified by HPLC or ion exchange
(compare with standards)
What reagents selectively react with N-terminus for analysis?
Sanger reagent - DNFB/FDNB
Dansyl chloride - DNSC
Edman reagent
What reagent selectively reacts with C-terminus for analysis?
Akabori procedure - hydrazinolysis
What is Sanger’s reagent?
Reacts at pH 8.5 with N-terminal amine via SnAr
Hydrolysis gives amino acids, with only terminal one being bound to reagent
What is the mechanism of the Sanger reagent?
What is dansyl chloride?
Identifies N-terminus
Via reacting with N-terminus of an amino acid (alternative to Sanger’s)
What is the mechanism of dansyl chloride derivitization?
What is the akabori procedure?
Identifies C-terminus amino acid
Requires addition of XS hydrazine and heat
What is the mechanism of the Akabori procedure?
What is Edman degredation?
Sequential removing of an N-terminus residue (one at a time) using limited acid hydrolysis or using enzymes
Used when large peptides
What is the mechanism of Edman degredation?
HPLC completed on these PTH derivatives against standards
Can then repeat on each N-terminus amino acid
What is required for protein sequencing with disulfide bonds?
Disulfide cross-links must be cleaved beforehand
How can you cleave a disulfide bond?
Reduction or oxidation then treatment to prevent disulfide bond reformation
What size of protein is required for Edman degradation?
10-20 residues
Larger must be fragmented using proteases then can do it
How can you selectively cleave at methionine residues?
Addition of BrCN
What does trypsin catalyse?
Peptide bond cleavage at +vely charged residues
(Arg, Lys)
Highly specific
What does chymotrypsin catalyse?
Peptide bond cleavage at bulky hydrophobic residues
(Phe, Trp, Tyr)
What does elastase catalyse?
Cleavage of peptide bond when small neutral amino acids
(Ala, Gly, Ser, Val)
What is nucleic acid sequencing (for protein sequencing)?
Find nucleic acid sequence of DNA
Then work out which amino acids this encodes for
This is efficient and most widely used modern method
How is X-ray crystallography & NMR used for protein structure?
Used for determining 3D structures of proteins
However crystallography doesnt work for disordered peptides and NMR is hard
What is tandem/fragmentation mass spec?
Peptides fragmented within mass spec and results are measured at each stage
Useful as accurate and post-translational modifications can be determined
What is the process of fragmentation/tandem mass spec?
1) Protein cleaved with protease (e.g. trypsin)
2) Compare with predicted fragments for protein sequences based on genomic data (automated)
3) Requires to be <25 residues
What fragmentation is seen in tandem mass spec of proteins?
Depends on sequences, charge, internal energy, etc.
Fragments only detected if charged
What is required for an amino acid protecting group?
- Easy to introduce
- Cleaved without amide bond hydrolysis or side-chain modification
- Must not lead to loss of stereochem (epimerisation)
What are the three types of protecting groups?
α - amino group
α - carboxylate group
Side-chain group
What is the general method for α-amino protection?
Alkoxycarbonyl protection
What are the α-amino protecting groups?
How do you protect an α-amino group (reagents)?
Need to add base to minimise side reactions
How can you remove protecting groups of α-amino groups?
Z: remove with Pd/H2 or HBr/AcOH
Boc: HBr/AcOH or TFA
Fmoc: base
What is the mechanism for removal of Z groups with Pd/H2 or HBr/AcOH?
How do you protect and deprotect an α-amino group with boc?
What is the problem with the boc PG when deprotecting?
Carbonium ion can be attacked by nuc side chains
(e.g. tyrosine)
Add anisole (Ph-OMe) as an anion scavenger
How do you protect an α-amino group with fmoc?
Fmoc-Cl and amino acid
(acyl chloride)
How do you remove a fmoc protecting group?
What is the general approach to protect α-carboxy groups?
Form their esters
Me/Et - too stable, so not used
Benzyl, tBu, allyl esters - more common
How can you protect and deprotect α-carboxyl group with benzyl esters?
Why is DMF used instead of DMSO in amino acid synthesis?
DMSO undergoes redox chemistry with side groups
How are benzyl esters deprotected?
H2/Pd/C
or
HBr/AcOH
How can you protect and deprotect a α-carboxy group with t-butyl?
What side chains typically need protecting?
Acidic: aspartic and glutamic acid (COOH)
Basic: lysine (NH2)
Cysteine (SH)
Not needed for alkyl chains
What side chains typically need protecting?
Acidic: aspartic and glutamic acid (COOH)
Basic: lysine (NH2)
Cysteine (SH)
Not needed for alkyl chains
How do you protect the lysine sidegroup?
Lys - amino group requires protection, is more nuc and basic than α-amino (which is zwitterionic)
Protect using CuSO4 then boc-anhydride and a base, then H2S