Ribosomes - Protein Synthesis Overview

Ribosomes - Location of Protein Synthesis

• Ribosomes are the sites of protein synthesis in cells.

Ribosome Structure

• A growing E. coli cell has ~10,000 ribosomes, each roughly 250nm in diameter.
• Ribosomes are composed of ribosomal RNA (rRNA) and ribosomal proteins. rRNA makes up about 2/3 of the ribosome's mass.
• Ribosomes are composed of two subunits: a large and a small subunit.
• Eukaryotic and prokaryotic ribosomes share similarities in structure.
• Ribosomes' most important functions are performed by RNA, demonstrating ribozyme activity.

Prokaryotic vs. Eukaryotic Ribosomes

• Ribosome sizes are measured using Svedberg units (S), which indicate the rate at which a particle sediments in a centrifuge and vary with structure.

Function of Ribosomes

• Ribosomes hold mRNA, aminoacyl-tRNA and translation factors in place for protein synthesis.
• Ribosomes catalyze chemical reactions in protein synthesis.

Ribosome Structure & Size

• Ribosomes have different sizes in prokaryotes and eukaryotes.
• Prokaryotes have 70S ribosomes (30S + 50S subunits), while eukaryotes have 80S ribosomes (40S + 60S subunits).
• The Svedberg value signifies sedimentation rate in a centrifuge, related to size.

Ribosome Binding Sites

• Ribosomes possess three binding sites: A (aminoacyl-tRNA), P (peptidyl-tRNA), and E (exit tRNA).

Ribosome Channels

• The small subunit has channels to allow mRNA entry and exit.
• The large subunit has channels where the growing polypeptide chain exits.
• Ribosomal size factors influence the shape and thus proper functioning of mRNA and polypeptide chains.

50S Ribosome Structure

• The 50S subunit is primarily composed of two RNA strands, with additional proteins holding the RNA strands in place.
• 50S ribosomes are rigid and immobile due to their tightly arranged structure.

30S Ribosome Structure

• The 30S subunit is fairly flexible, allowing for mRNA nucleotide triplet shifts.
• This flexibility facilitates accurate tRNA matching to mRNA codons and controls information flow during translation.
• mRNA enters the 30S ribosome via a channel.

Ribosome-Structure

• The 16S RNA subunit has a central domain, 5' domain, 3' major domain, and 3' minor domain.

Ribosome-Structure

• The 23S RNA subunit has multiple domains (I-VI).

Ribosome-Structure & Shape

• Ribosomes exhibit a variety of shapes. For example, the central region, neck, head, platform, and stalk portions are visible and define the three-dimensional structure.

Ribosome Function in Protein Synthesis

• Ribosomes are the primary structures responsible for synthesizing proteins from mRNA instructions via an assembly line approach.
• Ribosomes have three mRNA-binding sites for aminoacyl-tRNA, peptidyl-tRNA, and mRNA.

Ribosome Structure & Function

• The growing polypeptide chain extends from the ribosome and subsequently folds into the appropriate three-dimensional protein structure.

Ribozymes

• Ribozymes are ribonucleic acid enzymes, also known as catalytic RNAs.
• They exhibit catalytic activity similar to protein enzymes by catalyzing specific biochemical reactions.
• A 1989 Nobel Prize was awarded for the discovery of ribozymes within the introns of RNA transcripts.

Table 1: Naturally Occurring Ribozymes

• The table describes various ribozymes, their sequenced examples, sizes, and activities.

Ribosomes Are Not Ribozymes

• Ribosomes contain proteins that act like scaffolding to orient catalytic RNA for facilitating correct protein synthesis.
• Peptidyl transferase centers are comprised of RNA but not proteins.
• The RNA itself is responsible for the enzyme activity within the ribosome.

This quiz covers the structure and function of ribosomes, highlighting their role in protein synthesis within prokaryotic and eukaryotic cells. Understand the differences between prokaryotic and eukaryotic ribosomes, including their size and structure.