Week 7 proteins structure and synthesis Flashcards
Protein functions
> 50% of the cells mass
-Enzymatic
-Transport
-Defensive roles
-Regulation
-Structural support
-Receptor molecules
-Hormones
-Cell signalling
-Amino acid storage
-Movements within cells
Protein features
Linear chains of amino acids, arranged in a 3D structural hierarchy
Protein features 2
Diversity of structures, resulting in a wide range of
a functions:
▪ functions depend on the 3D protein shape
▪ 3D protein shape is determined by its
amino acid sequence
Without its normal conformation (native folding), the protein loses its biological activity
Amino acids
Monomers
Proteins are assembled from a set of 20 different amino acids
Amino acid structure
Amino acids have in common: Carbon α (Cα), amino
(-NH2) and carboxyl (–COOH) functional groups
R group or side chain is specific for each amino acid
Polypeptides
Protein chains
In a protein, amino acids are linked together by covalent peptide bonds formed between carboxylic acid and amino groups of two consecutive amino acids
Polypeptides 2
-Form linear, unbranched polypeptides (backbone of the protein)
-Every protein has a unique sequence of amino acids with a carboxyl end (C-term) and an amino end (N-term)
4 levels of protein structure
The architecture of proteins has four levels of organisation:
1) Primary - amino acids
2) Secondary - α-helix / β-pleated sheet
3) Tertiary
4) Quaternary
Primary structure
The unique sequence of amino acids in a polypeptide chain,
▪ Sequence is determined by the DNA of the gene that encodes the protein
▪ Linked by peptide bonds
-Peptide = short chain (<50 amino acids)
-Protein = longer chain (>50 amino acids).
Also referred to as polypeptides
Secondary structure
Local and folded structures that form within a polypeptide due to:
- H- bonds between backbone of amino and carbonyl groups of two amino acids
Forms 2 structural patterns:
α-helix and β-pleated sheet
Tertiary structure
Tertiary structure is the three-dimensional folding of a polypeptide
▪ determined by interactions among various side chains
Quaternary structure
Quaternary structure is the result of the association of two or more polypeptide
chain subunits into a closely packed arrangement
Protein Synthesis in eukaryotic mammalian cells
Protein synthesis (or gene expression) consists in two phases - transcription
and translation (separated by the RNA maturation in eukaryotic cells)
Transcription - in the nucleus
Translation - By ribosomes in the cytoplasm
DNA transcription - key points
DNA to mRNA
-In eukaryotic cells, it occurs in the nucleus
-RNA polymerase uses one of the DNA strands as a template to produce pre-messenger RNA (pre-mRNA) with a complementary sequence to DNA.
-It adds RNA nucleotides (complementary to DNA) in the 5′→3′ direction
Consists of 3 phases:
-Initiation
-Elongation
-Termination
Transcription - initiation
-RNA polymerase, with the help of transcription factors, binds to a specific sequence of DNA (promoter – just before the start site of a gene)
-RNA polymerase separates the double strands and use a single strand as a template
-Begins the complementary RNA strand synthesis
Transcription - elongation
-RNA polymerase moves along the DNA coding sequence in 5ʹ-to-3ʹ direction.
-Melting DNA and adding RNA nucleotides
Transcription - termination
The RNA polymerase encounters a specific termination sequence (stop site) -> this releases the RNA strand
The result of transcription is the mRNA precursor (pre-mRNA)
U replaces T
In the RNA strand the bases are complementary to the DNA strand, however, Uracil takes the place of Thymine to pair with A
E.g.,
DNA -> TAC GCA TGA CTA
RNA -> AUG CGU ACU GAU
RNA processing
Pre-mRNA must undergo RNA processing steps:
-Capping at 5’ end
-Polyadenylation at 3’ end
-Splicing
Capping at 5’ end
A modified nucleotide 5′ G (cap), is added to the 5’ of the pre-mRNA
▪ Allows the attachment to the ribosome
Polyadenylation
A repetition of poly-A (100–250 bp of ribonucleotide A) is added to the 3’ end
▪ increase the stability of mRNA molecules (preventing its degradation)
▪ facilitates its migration to the cytosol
Splicing
Not all sequences of a pre-mRNA hold instructions for proteins:
* INTRONS – internal sequences that do not code for proteins
* EXONS – segments that do code for proteins -> expressed
Splicing 2
Introns are cut out and exons are joined together
After RNA processing, pre-mRNA becomes a mature mRNA → in cytoplasm
Translation - key points
Translation – conversion of the mRNA code (4 nucleotides) into protein (20 amino acids)
* In the cytoplasm on the ribosomes
-mRNA carries the genetic code in groups of three consecutive
nucleotides → Codons
* Each codon specifies one amino acid (Universal Genetic
Code), except 3 stop codons
-Some amino acids are specified by more than one codon
