Quick Review of Protein Synthesis

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1 Collin College BIOL Quick Review of Protein Synthesis. Proteins and Protein Synthesis Proteins are the molecular units that do most of the work in a cell. They function as molecular catalysts, help in transport, signal transduction, cell recognition, defense mechanism and in various other homeostatic activities. Regulation of protein synthesis is thus of crucial importance for a cell. Manipulation of these processes or the repair of malfunctioning processes is now possible at the molecular level thanks to the technology of molecular biology and genetic engineering. 1

2 Protein Synthesis Review o The genome ( genomic DNA ) contains the genetic information o The specific information that specifies the amino acid sequence in a protein is contained within the linear sequence of the nucleotide bases within a DNA gene o Each amino acid is represented by 3 nucleotide bases within the DNA (= triplet codon) o DNA does not participate directly in the making of a protein ; active DNA genes produce messenger RNA which migrate to the cytoplasm where they generate proteins with the help of ribosomes and transfer-rna o The process of transferring DNA information into mrna = transcription o The process of making a protein from mrna = translation Protein Synthesis Review 2

3 DNA : double helix molecule Nucleotides: (sugar, phosphate, base) Nucleotide Sequence Information Triplet Codon messenger RNA (mrna) passes the information from DNA to the cytoplasm before a protein can be made 3

4 DNA : double helix molecule Gene Activation is the process where the information within a gene becomes expressed. That information usually means the production of a protein with a specific function. Gene activation requires the uncoiling of DNA to use it Copying the instructions from DNA to mrna (known as Transcription, which occurs in thenucleus) Transcription involves the RNA polymerase, which produces messenger RNA (mrna) DNA to mrna : Transcription The process of Transcription requires Transcription Factors ; they are a class of proteins that can bind to DNA, the promotor site or to RNA polymerase They will start or inhibit the process of transcription The promotor site is where the RNA polymerase starts reading 4

5 Transcription factors This Transcription Factor Prevents the reading of Gene A This one Activates the reading of Gene B PostTranscriptional Process A mrna contains DNA information with relevant and non-relevant (nonsense) information The portion of DNA with relevant information = EXON The portion of DNA that is not needed = INTRON The initial mrna will contain these transcribed portions of introns and exons = pre-mrna Splicing is the process in which introns and spliced out of the mrna and exons are joined together Splicing is handled by Spliceosomes in the nucleus The functional mrna moves into the cytoplasm through the nuclear pores and into the rough ER 5

6 mrna Splicing = removed Protein Synthesis = Translation Once in the cytoplasm, a functional polypeptide is constructed at Ribosomes, using the information in the mrna codons The linear sequence of the codons determines the sequence of amino acids Specific Transfer RNA s (trna) bring sequential Amino Acids to the ribosomal complex according to their complementary base pairing of anticodons 6

7 Transfer RNA and Anticodon Amino Acid mrna to Protein : Translation Certain small proteins known as ribosomal initiation factors are required to establish an " initiation" complex for translation! 7

8 mrna to Protein : Translation When ribosomes reach the stop codon, the system falls apart and the protein is complete. In this example, the completed protein has 3 amino acids. Protein Synthesis The longer a mrna, the more ribosomes can fit onto it and read it, the more protein can be made per time unit. The picture below show plenty of ribosomes reading a mrna lots of protein production going on there! 8

9 PostTranslational Process Once a protein is made, it may not be functional yet. The process of making a protein functional is called PostTranlational modification. There are many ways this can happen. This can include removal of precursor elements or signaling elements. Insulin for example is first created as a pre-pro-insulin protein. Removal of the precursor signaling peptide, results in pro-insulin. Proinsulin is modified and cleaved again, yielding active insulin Some mrna s create proteins that are actually 2 or 3 functional proteins. By cleaving the initial protein, 2 or 3 functional proteins are created. PostTranslational Process 9

10 Overall Control of Protein Activity The action-effect of a protein depends on its activity and on its concentration. Concentration is dependent on synthesis, stabilization and degradation and as such, can be affected by modulation of these processes. Synthesis Stabilization [Protein] Degradation Modulation Factors that determine Protein Levels Transcription of DNA o Transcription factors can activate or repress the transcription process o Activating or inhibition of transcription factors can thus result in more or less mrna Splicing of mrna o The activity of the spliceosomes will determine how many "active" mrna will be produced, and thus how functional the protein production will be. mrna regulation o RNAses are enzymes that break down mrna. o Longevity of the mrna depends on factors that resist the mrna from being broken down. o The longer a mrna molecule exists, the more protein can be produced from that mrna. 10

11 Factors that determine Protein Levels Translation of the mrna o Certain small proteins known as ribosomal initiation factors are required to establish an " initiation" complex for translation. o The effectiveness of the ribosome initiation complex with the mrna determines how effective the protein production will be. Protein degradation o Every proteins has a certain life span and is subject to degradation o Proteins are targeted for degradation by the attachment of a small protein called Ubiquitin. o This protein directs it to a proteasome which breaks down the protein o Proteins can be protected by other proteins that prevent them from breakdown and unfolding ( e.g. heatshock proteins). Factors that determine Protein Levels Allosteric and covalent modification o This process does not change the effective concentration of a protein o It modulates ( = changes) the activity of a protein by inhibiting or increasing its efficiency Allosteric modulation Due to a the binding of a modulator molecule at a site away from the active site This other binding site is known as the regulatory site The result of a modulator molecule binding to the regulatory site, is that it changes the conformation of the active site and thus alters the properties of the active site. 11

12 Factors that determine Protein Levels Covalent modification The activity of a protein is changed by the covalent bonding of a chemical group to the protein Most common way is phosphorylation : it is the attachment of a phosphate group to a protein. The phospate group usually comes from ATP Phosphorylation affects the ligand binding properties of the protein. Enzymes that mediate phosphorylation are called protein kinases Enzymes that remove the phosphate are called (phospho)-protein phosphatases. Allosteric Vs. Covalent Modulation 12

13 Factors that determine Protein Levels 13