The Vital Role of Ribosomes in Mitochondria and Skin Cells .

By Gay Wardle

Ribosomes are essential molecular machines within cells, they are responsible for protein synthesis. They are found in all cell types, including the mitochondria and skin cells, They play a crucial role in cellular function and health.

The composition of Ribosomes is made up of ribosomal RNA (rRNA) and proteins.

A subunit is a distinct component of a larger complex, often a protein complex. In the context of ribosomes, a subunit refers to one of the two structural units that come together to form the complete ribosome. Ribosomes are made up of two subunits, each with its own specific roles and characteristics The two subunits are - small subunit is 40S in eukaryotes and the large subunit 60S in eukaryotes.

What is the function of Ribosomes?

The role of Ribosomes is to translate messenger RNA (mRNA) into proteins by facilitating the binding of transfer RNA (tRNA) and the incorporation of amino acids into the growing polypeptide chain.

For the Ribosomes to be able to translate messenger RNA (mRNA) into proteins involves several key steps. These steps are Initiation, Elongation, and Termination.

Let`s take a look at each of these three parts to each step.

Initiation – Formation of the Initiation Complex:

1. mRNA Binding: The small ribosomal subunit binds to the mRNA at the 5’ end. In eukaryotes, this process involves recognition of the 5’ cap structure of the mRNA. In prokaryotes, the ribosome binds to a specific sequence called the Shine-Dalgarno sequence.

2. Initiator tRNA: A special initiator tRNA, carrying the amino acid methionine (in eukaryotes) or formylmethionine (in prokaryotes), binds to the start codon (AUG) on the mRNA. This initiator tRNA binds to the P (peptidyl) site of the small ribosomal subunit.

3. Assembly of the Ribosome: The large ribosomal subunit then joins the complex, forming the complete ribosome. This assembly creates three sites within the ribosome: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site.

Elongation – Amino Acid Incorporation:

1. tRNA Entry: A tRNA carrying the next amino acid in the sequence binds to the A site of the ribosome. This tRNA has an anticodon that is complementary to the mRNA codon at the A site.

2. Peptide Bond Formation: The ribosome catalyses the formation of a peptide bond between the amino acid attached to the tRNA in the P site and the amino acid attached to the tRNA in the A site. This reaction transfers the growing polypeptide chain to the tRNA in the A site.

3. Translocation: The ribosome moves one codon down the mRNA in a process called translocation. This shifts the tRNA in the A site to the P site and the tRNA in the P site to the E site, where it is released from the ribosome. The A site is now free to accept the next tRNA.

Termination – Completion of Translation:

1. Stop Codon Recognition: When a stop codon (UAA, UAG, or UGA) is encountered on the mRNA, no corresponding tRNA binds to the A site.

2. Release Factors: Release factors bind to the ribosome, prompting the release of the polypeptide chain from the tRNA in the P site.

3. Disassembly: The ribosomal subunits, mRNA, and release factors dissociate, ready to participate in another round of translation.

These steps collectively ensure the accurate translation of genetic information from mRNA into a functional protein, essential for cellular activities.

Ribosome production and the Nucleus.

The nucleus plays a crucial role in the production of ribosomes. The nucleolus, a specialized region within the nucleus, is the site where ribosomal RNA (rRNA) is transcribed, and ribosome assembly begins. Within the nucleolus, genes encoding rRNA are transcribed by RNA polymerase I to produce large precursor rRNA molecules. Once the ribosomal subunits are assembled, they are transported out of the nucleus through the nuclear pores into the cytoplasm. In the cytoplasm, the small and large subunits join together during translation to form functional ribosomes.

In summary, the nucleus, and more specifically the nucleolus, is the site of ribosome biogenesis. It orchestrates the production and initial assembly of ribosomal subunits, which are essential for protein synthesis in the cytoplasm. Therefore, while ribosomes do not have a direct function in the nucleus, the nucleus is indispensable for their creation and proper functioning.

Functions of Mitochondrial Ribosomes.

Mitochondrial ribosomes are responsible for translating the 13 essential protein-coding genes encoded by the mitochondrial DNA (mtDNA). These proteins are integral components of the oxidative phosphorylation system.

The proteins synthesized by mitoribosomes are crucial for the assembly and function of the respiratory chain complexes (Complex I, III, IV, and V) within the inner mitochondrial membrane. The proteins produced by mitoribosomes include subunits of ATP synthase (Complex V), which is responsible for the production of ATP through oxidative phosphorylation. ATP is the primary energy currency of the cell. Mitoribosomes synthesize proteins that are part of the electron transport chain, which creates a proton gradient across the inner mitochondrial membrane. This gradient drives the synthesis of ATP.

By synthesizing key proteins, mitoribosomes help maintain the integrity and function of the mitochondrial genome. This is essential for the replication and expression of mitochondrial DNA. Mitoribosomes allow mitochondria to adapt to varying cellular energy demands. Under conditions that require increased energy production, mitoribosomes can enhance the synthesis of mitochondrial proteins to meet the higher energy requirements.

The primary functions of ribosomes in the mitochondria are centred around the synthesis of proteins that are essential for mitochondrial function, energy production, and overall cellular metabolism. These functions are critical for maintaining the health and efficiency of eukaryotic cells, as mitochondria are the powerhouse of the cell, providing the necessary energy for various cellular processes.

The Rough Endoplasm Reticulum.

Ribosomes on the rough ER are responsible for synthesizing proteins that are destined for secretion out of the cell, incorporation into the cell membrane, or delivery to lysosomes. These proteins typically have a signal sequence at their N-terminus, which directs the ribosome to the ER membrane. This sequence is recognized by the signal recognition particle (SRP), which pauses translation and directs the ribosome to the ER.

Once the ribosome-SRP complex binds to the SRP receptor on the ER membrane, the ribosome resumes translation, and the growing polypeptide chain is translocated into the lumen of the ER through a protein-conducting channel called the translocon. Within the ER, the nascent proteins undergo proper folding and post-translational modifications, such as glycosylation and disulfide bond formation, which are essential for their function and stability.

By localizing ribosomes to the ER, the cell ensures that proteins entering the secretory pathway are immediately in the right location for proper processing and sorting. Proteins that are integral membrane proteins are inserted directly into the ER membrane during translation, facilitating their correct orientation and insertion. The ER contains quality control mechanisms to ensure that only properly folded proteins proceed along the secretory pathway. Misfolded proteins are retro-translocated to the cytoplasm for degradation by the proteasome. Molecular chaperones within the ER assist in the proper folding and assembly of newly synthesized proteins.

The key differences between, Mitochondrial Ribosomes, Endoplasm Ribosomes and Nucleus Ribosomes are: Mitochondrial Ribosomes have a unique structure where they are smaller and have a higher protein-to-RNA ratio. They synthesize proteins encoded by mitochondrial DNA that are essential for mitochondrial function, particularly components of the electron transport chain and ATP synthase. They Translate mitochondrial mRNA into proteins critical for energy production through oxidative phosphorylation.

Endoplasmic Reticulum Ribosomes are attached to the cytoplasmic side of the rough endoplasmic reticulum. They are part of the larger cytoplasmic ribosomal pool; indistinguishable from free ribosomes in terms of structure. They Synthesize proteins that are destined for secretion, incorporation into the cell membrane, or delivery to lysosomes. They are involved in co-translational translocation where nascent polypeptides are directly translocated into the ER lumen for proper folding, modification, and sorting.

Nucleus and Ribosome are not found functioning within the nucleus, but the nucleus (specifically the nucleolus) is crucial for ribosome biogenesis. The nucleolus within the nucleus is the site where ribosomal RNA (rRNA) is transcribed, and initial ribosomal subunit assembly begins. Precursor rRNA is processed and combined with ribosomal proteins imported from the cytoplasm to form ribosomal subunits. Ribosomal subunits are exported to the cytoplasm where they combine to form functional ribosomes involved in cytoplasmic and rough ER-associated protein synthesis.

Conclusion:

Mitochondria are indispensable for skin health due to their roles in energy production, metabolic activities, ROS management, apoptosis regulation, cellular signalling, stem cell maintenance, and barrier function. Proper mitochondrial function ensures that skin cells have the energy and resources needed for growth, repair, and protection, ultimately contributing to overall skin health and appearance. Understanding and maintaining mitochondrial health is thus crucial for healthy, youthful skin.

@gaywardle