Mitochondrial Protein Synthesis

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

Mitochondria, the miniature powerhouses of the cell, act like tiny batteries to give us energy and to utilize oxygen. In this lesson we'll discuss how mitochondria make their proteins, and how these proteins are used.

The Powerhouse Of The Cell

Imagine thousands of tiny batteries, buzzing with energy, giving life to the engines they run. Zoom out a bit, and the engines are living cells in a body. These tiny batteries are mitochondria (singular mitochondrion), and they are known as the powerhouse of the cell - for good reason.

Among all of the organelles, the miniature functioning parts of a cell, mitochondria may be the most important. Without mitochondria, we wouldn't be able to process the nutrients from the food we eat, or use the oxygen in the air we breathe.

Mitochondria take in energy from the breakdown of sugars we eat in our food, and they use up the oxygen we breathe, converting it to waste carbon dioxide. In return, this breakdown process provides a burst of cellular energy in the form of ATP, or adenosine triphosphate. In order to make this life-sustaining energy molecule, mitochondria need to create, or synthesize, a series of proteins to get the job done.

An electron microscope image of two mitochondria, with the characteristic folds and creases of this organelle
mitochondrion

Think of these mitochondrial proteins as the wheels and gears inside a machine. Without the right cogs set up in the right places, the machine won't run. And without the right proteins in the proper order, mitochondria can't make the energy molecule that powers the cell and, in turn, powers a living organism.

How are mitochondrial proteins made? Where are they made? And how do they function? Let's delve into the process of mitochondrial protein synthesis, and touch on the function of these proteins.

Mitochondrial Protein Synthesis

Before we discuss how mitochondrial proteins are made, let's lay the groundwork with a little cellular terminology.

Most mitochondrial proteins, as many as 99%, are made outside of the mitochondrion itself, in the liquid cytoplasm that fills the entire cell. Mitochondria float within this cytoplasm, along with the rest of the organelles that make the cell run smoothly.

The internal anatomy of a mitochondrion
mitochondrion

Other mitochondrial proteins are made inside the mitochondrion, within the organelle's own liquid-filled interior compartment known as the mitochondrial matrix. The matrix contains the folds and pockets, called cristae, where chemical reactions occur within the mitochondrion. Surrounding the mitochondrion are an inner membrane and outer membrane. The thin space in between the inner and outer mitochondrial membrane is known as the intermembrane space.

Proteins that are made outside the mitochondrion in the cellular cytoplasm undergo the process of translation from messenger RNA to the individual amino acids that make up the protein chain. These proteins are then shuttled to the mitochondrion through special cellular signals that match the proteins to the outer membrane surface.

From the surface, the proteins interact with a protein called translocase, which sorts the proteins toward their final destination inside the mitochondrion. Extra-mitochondrial proteins can end up being folded into the outer or inner membranes, trapped inside the intermembrane space, or sorted within the interior matrix of the mitochondrion.

Inside the mitochondrion, only a small fraction of proteins are made. These matrix proteins might not be numerous, but they perform critical functions inside the organelle. Parts of the mitochondrial ribosomes, small complexes of RNA and protein that translate mitochondrial RNA into proteins, are made within the mitochondrion. Another important protein made within the mitochondrion is a portion of the protein complex that makes ATP, known as ATP synthase.

Mitochondrial Protein Function

In certain yeasts, there are more than one thousand proteins encoded for mitochondria to function properly. Most of these proteins are transported from the yeast's cytoplasm into the cellular mitochondria. About 25% of these proteins function in expressing and maintaining pieces of DNA within the mitochondrian genome. In fact, the gene upkeep seems to focus strongly on maintaining the integrity of the small percentage of proteins that are made within the mitochondrial matrix.

Moving proteins across cell membranes and protein folding are coordinated by about 10% of the proteins in this same yeast example. The function of about 20% of mitochondrial proteins is still unknown.

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