Denaturation

Protein Denaturation

Let's start by looking at the definition of proteins.

Proteins are essential to life as they perform a variety of functions. They can transport materials, control physiological processes such as growth, speed up chemical reactions, store things, etc.

Figure 1: Amino acid structure. Daniela Lin, Study Smarter Originals.

Featured on the table below are some common examples of proteins:

Proteins come in different sizes and shapes, and the shape of proteins is essential for their functions. There are generally two shapes of proteins: globular and fibrous.

• Globular proteins are spherical, usually act as enzymes or transport materials, are generally soluble in water, have an irregular amino acid sequence, and are usually more sensitive to heat and pH changes than fibrous ones. A globular protein is a hemoglobin, as shown in Figure 2.

• Fibrous proteins are narrower and more prolonged, usually are structural in function, are generally not soluble in water, have a regular amino acid sequence, and are usually less sensitive to heat and pH changes than globular ones. An example of a fibrous protein is keratin, as shown in Figure 2.

Figure 2: Examples of different protein shapes. Daniela Lin, Study Smarter Originals.

Globular proteins are more soluble than fibrous because the amino acids can bend in a way where the polar groups are on the surface. Globular proteins also have weaker interactions between their amino acid sequences when compared to fibrous proteins making them easier to denature.

When a protein loses its shape, we say it denatures.

Denaturation often results in loss of protein shape, structure, function, and eventual degradation.

Causes of Denaturation

Now that we know what protein denaturation is, we can ask what causes a protein to denature.

• Temperature changes or heat increases result in molecule vibrations that break bonds in proteins leading to denaturation. Note that denaturation because of cold temperature also occurs; it's harder to detect as it happens below freezing temperatures or below \(0^\circ C\).

• pH changes lead to an acid-base imbalance or different amounts of hydrogen and hydroxide ions which can affect the bonding in proteins resulting in denaturation.

Other physical and chemical changes in a protein's environment can also lead to denaturation:

• Addition of chemical agents, such as urea. Urea leads to protein denaturation because it forms stronger hydrogen bonds with the protein backbone than water does.

• Addition of metal poisons like lead. Metals like lead interact with a protein's calcium-binding sites leading to the breakage of bonds or denaturation.

All common factors that can cause denaturation are shown in the table below:

Characteristics of Denaturation

After understanding what protein denaturation is and why proteins denature, we must go over what parts of a protein can denature. We find this by understanding how a protein's final conformation is created.

As we discussed earlier, proteins are made of a chain of amino acids. When a few chains of amino acids bind together, they create peptide bonds. Longer chains of amino acids bound together are called polypeptide bonds. All structures mentioned below are shown in Figure 3 except the (\(\beta\)) pleated sheets.

• Primary structure: A protein's primary structure is its amino acid sequences linked into a polypeptide chain. This sequence determines a protein's shape.

• Secondary structure: The secondary structure is caused by folding amino acids from the primary structure. The most common structures proteins fold into in the secondary level are alpha (\(\alpha\)) helices and beta (\(\beta\)) pleated sheets, which are held together by hydrogen bonds.

• Tertiary structure: The tertiary structure is a protein's three-dimensional structure. This three-dimensional structure is formed by the interactions between the variable R groups.

• Quaternary structure: Not all proteins have a quaternary structure. But some proteins can form quaternary structures that consist of multiple polypeptide chains. These polypeptide chains can be referred to as subunits

Figure 3: Protein structure (primary, secondary, tertiary, and quaternary). Daniela Lin, Study Smarter Originals.

Each protein has its own sequence and shapes with chemical interactions that bond it together. Denaturation occurs when there are physical and chemical changes to the environment, including but not limited to temperature, pH, and the addition of chemicals. Denaturation often involves the breakdown of every structure except the primary structure.

In general, denatured proteins have a less uniform, loose structure which is usually insoluble in water.

The denaturation of proteins destroys a protein's three-dimensional or tertiary structure and secondary structure (also quaternary if a protein has more than one polypeptide chain).

Whether a protein can fold back after denaturation depends on the complexity and type of the protein itself. The reason "renaturation" can occur is that the primary structure of a protein isn't destroyed. Some proteins have all the instructions needed for folding in their amino acid sequence. However, other proteins might need chaperones.

However, in extreme conditions, renaturation isn't possible. For instance, when we boil or fry an egg, we put it through so much heat that renaturation of the protein can't occur.

Significance of Protein Denaturation

When proteins denature, they stop functioning, and this is because the protein's shape and conformation determine a protein's function.

When proteins stop functioning, they can't transport materials, signal molecules, support structures in our bodies, and more! Proteins also help repair and make new cells essential for survival.

But besides being the building blocks of our lives, proteins can also be used in novel drug designs. Designing drugs involves designing molecules whose shapes complement our biological target. This is because drugs usually work by binding to the target's receptor site or where they either block the protein's effect or copy it. Sometimes drugs can bind to multiple targets' receptor sites.

Scientists think that by binding to multiple targets' receptor sites, drugs can create a "molecular interaction signature" that can be used to identify new drugs or new uses for old/existing drugs.

Denaturation Examples

Now, let's explore some examples of denaturation.

Eggs

Eggs are made of both water and protein. We're denaturing the protein halfway when we whip egg whites to get air into them, creating a light and fluffy consistency perfect for desserts. If the egg whites get beaten, they will eventually denature all the way and become stiff.

Molecularly, when the egg whites are being beaten, the protein unravels so that the hydrophilic ends bond with water, and the hydrophobic parts go towards the air. This creates bonds that hold the whipped mixture and air bubbles in place. If they aren't beaten until stiff, then the proteins are only half-denatured, which means they will retain their elastic properties. When baked, the proteins will denature completely and surround the air bubbles trapping them, thereby creating a light and fluffy consistency in some desserts like souffle.

Meat

Cooking meat involves denaturation by heat, making it not reversible at all. This process accompanies color changes. For example, when we cook a steak, structural changes happen. The connective tissues and muscle proteins start to denature, changing meat color and tenderness.

But precisely what are some proteins involved in this process?

• Myoglobin is what gives meat its reddish hue. Due to myoglobin denaturation, the color changes around 60 \(^\circ C\).

• Meat changes from see-through to opaque as the proteins denature because the protein unravels and coagulates, making light unable to pass.

• Myosin, a fibrous protein, is responsible for the changes in meat texture from raw to tender around 40 \(^\circ C\).

• Actin, another fibrous protein, denatures at higher temperatures than myosin. It denatures around ~70 \(^\circ C\), and it's responsible for making meat tougher and drier.