Protein Synthesis

Protein Synthesis

From: Protein Data Bank PDB ID: 1A3N
Tame, J., Vallone, B.: Deoxy Human Hemoglobin. 1998
Nucleic Acids
Nucleic acids made up of chains of nucleotides
Nucleotides consist of:
A base
A sugar (ribose)
A phosphate
Two types of nucleic acids in cells:
Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)
Adapted from: Bettelheim FA and March J (1990) Introduction to Organic and Biochemistry (International Edition). Philadelphia: Saunders College Publishing p383.
Nucleic Acids
Nucleic acids have primary and secondary structures
DNA
Double-stranded helix
H-bonds between strands
RNA
3 kinds (mRNA, tRNA, rRNA)
All single strands
H-bonds within strands
From: Bettelheim FA and March J (1990) Introduction to Organic and Biochemistry (International Edition). Philadelphia: Saunders College Publishing p391 (Left panel) and 393 (Right panel).
Complementarity of bases
The different bases in the nucleotides which make up DNA and RNA are:
Adenine
Guanine
Cytosine
Thymine (DNA only)
Uracil (RNA only)
Chemical structure only allows bases to bind with specific other bases due to chemical structure
From: Elliott WH & Elliott DC. (1997) Biochemistry and Molecular Biology. New York: Oxford University Press. P245.
Table showing complementarity of base pairs
* Present only in DNA
**Present only in RNA
DNA
DNA
Located in 23 pairs of chromosomes in nucleus of cell
DNA has two functions:
Replication - reproduces itself when cell divides
Information transmission
via protein synthesis
From: Tortora, GJ & Grabowski SR (2000) Principles of Anatomy and Physiology (9th Ed). New York: John Wiley & Sons. P86.
DNA
DNA contains genetic information
Gene - segment of DNA on a chromosome that codes for a particular protein
Coding contained in sequence of bases (on mRNA) which code for a particular amino acid (i.e. genetic code)
Genetic code universal in all organisms
Mitochondrial DNA slightly different
From: Elliott WH & Elliott DC. (1997) Biochemistry and Molecular Biology. New York: Oxford University Press. P294.
RNA
Four types of RNA:
Messenger RNA (mRNA) - carries genetic information from DNA in nucleus to cytoplasm where proteins synthesised
Transfer RNA (tRNA) - carries amino acids from amino acid pool to mRNA
Ribosomal RNA (rRNA) - joins with ribosomal proteins in ribosome where amino acids joined to form protein primary structure.
Small nuclear RNA (snRNA) - associated with proteins in nucleus to form small nuclear ribonucleoprotein particles (snRNPs) which delete introns from pre-mRNA
Information transmission
Information stored in DNA transferred to RNA and then expressed in the structure of proteins
Two steps in process:
Transcription - information transcribed from DNA into mRNA
Translation - information in mRNA translated into primary sequence of a protein
Transcription
Information transcribed from DNA into RNA
mRNA carries information for protein structure, but other RNA molecules formed in same way
RNA polymerase binds to promoter nucleotide sequence at point near gene to be expressed
DNA helix unwinds
RNA nucleotides assemble along one DNA strand (sense strand) in complementary sequence to order of bases on DNA beginning at start codon (AUG - methionine)
Transcription of DNA sense strand ends at terminator nucleotide sequence
mRNA moves to ribosome
DNA helix rewinds
From: Tortora, GJ & Grabowski SR (2000) Principles of Anatomy and Physiology (9th Ed). New York: John Wiley & Sons. P88.
Transcriptional control
Each cell nucleus contains all genes for that organism but genes only expressed as needed
Transcription regulated by transcription factors
Proteins produced by their own genes
If transcription factors promote transcription - activators
If transcription factors inhibit transcription - repressors
General transcription factors interact with RNA polymerase to activate transcription of mRNA
Numerous transcription factors required to initiate transcription
General transcription factors set base rate of transcription
Specific transcription factors interact with general transcription factors to modulate rate of transcription
Some hormones also cause effects by modulating rate of gene transcription
Regulation of transcription in skeletal muscle
Ca2+ initiates contraction
Cytoplasmic Ca2+ concentration reflects frequency and duration of fibre activation
Calcium binds to calmodulin (CaM)
Ca2+-CaM complex binds to calcineurin (a protein phosphatase)
Calcineurin dephosphorylates transcription factor called nuclear factor of activated T cells (NFAT)
NFAT first identified in T cells, but also found in skeletal muscle
NFAT binds to response element in nucleus
Response element regulates gene transcription
Increases expression of genes for myogenic regulatory factors
influence synthesis of myosin light and heavy chains
From: Houston ME (2001) Biochemistry Primer for Exercise Science. Champaign: Human Kinetics, p168.
Translation (protein synthesis)
Information in mRNA translated into primary sequence of a protein in 4 steps:
ACTIVATION
INITIATION
ELONGATION
TERMINATION
Translation (protein synthesis)
ACTIVATION
Each amino acid activated by reacting with ATP
tRNA synthetase enzyme attaches activated amino acid to own particular tRNA
Adapted from: Bettelheim FA and March J (1990) Introduction to Organic and Biochemistry (International Edition). Philadelphia: Saunders College Publishing p398
Translation (protein synthesis)
INITIATION
mRNA attaches to smaller body of ribosome
Initiator tRNA attaches to start codon
Larger body of ribosome combines with smaller body
From: Tortora, GJ & Grabowski SR (2000) Principles of Anatomy and Physiology (9th Ed). New York: John Wiley & Sons. P88.
Translation (protein synthesis)
ELONGATION
Anticodon of next tRNA binds to mRNA codon at A site of ribosome
Each tRNA specific for one amino acid only, but some amino acids coded for by up to 6 codons
Order of bases in mRNA codons determine which tRNA anticodons will align and therefore determines order of amino acids in protein
Amino acid at A site linked to previous amino acid
Ribosome moves along one codon and next tRNA binds at A site
From: Tortora, GJ & Grabowski SR (2000) Principles of Anatomy and Physiology (9th Ed). New York: John Wiley & Sons. P88.
Translation (protein synthesis)
TERMINATION
Final codon on mRNA contains termination signal
Releasing factors cleave polypeptide chain from tRNA that carried final amino acid
mRNA released from ribosome and broken down into nucleotides
From: Tortora, GJ & Grabowski SR (2000) Principles of Anatomy and Physiology (9th Ed). New York: John Wiley & Sons. P88.
Control of protein synthesis
Rate of protein synthesis:
suppressed during exercise
increases for up to 48 hours post-exercise
Increased protein synthesis during post-exercise period
unlikely to be due to increased transcription of RNA
Changes in protein synthesis independent of total RNA
more likely due to change in translational control of mRNA
Recent evidence points to involvement of translational initiation factors (eIF4E & eIF4G)
Extent of post-exercise protein synthesis also dependent on half-life of mRNA
Controlled by ribonucleases (degradative enzymes)
Other proteins stabilise and destabilise mRNA against degradation by ribonucleases
Mitochondrial protein synthesis
Mitochondria contain own DNA and protein synthesizing machinery
Mitochondrial genetic code slightly different
Codon-anticodon interactions simplified
Manage with only 22 species of tRNA
Synthesise only small number of proteins
Most mitochondrial proteins coded for in nucleus and transported into mitochondria
Adapted from: Tortora, GJ & Grabowski SR (2000) Principles of Anatomy and Physiology (9th Ed). New York: John Wiley & Sons. P84.
Protein degradation
Protein content of a cell depends on balance between protein synthesis and degradation
Change in protein = synthesis rate - degradation rate
Protein degradation
Three main protein degrading systems in muscle:
Ubiquitin-proteosome
Protein marked for degradation by attachment of ubiquitin units
Inactive 20S proteosome activated by regulatory protein to become active 26S proteosome
26S proteosome breaks protein into small peptides
Small peptides broken down into free amino acids by other processes in cell
Lysosomal
Proteins enter lysosome via endocytosis
cathepsins and proteinases degrade bonds
Calpain
Calcium activated proteinase in cytosol of cell
Various isomers activated at different calcium concentrations