Proteins are macromolecules that are made up of carbon, hydrogen, oxygen and nitrogen. They may also contain sulfur. Proteins are considered the building blocks of tissue in living things. They are occasionally used as a fuel source after carbohydrates and lipids.
Proteins perform several critical jobs in cells. Functions of proteins include:
- regulating cell processes
- forming structural components of certain cells
- transporting substances into and out of cells
- healing to fight disease
- controlling the rate of chemical reactions (enzymes)
Monomer units of proteins are called amino acids. There are 20 types of amino acids that exist in nature. They all have the same general structure that you see in the image below. Each amino acid consists of the following parts: central carbon atom, amino group (NH2), acid group (COOH) and R group (where R is one of 20 other groups of atoms). Visit the link in the sidebar to view an image of all 20 amino acids and get an idea of how varied their R groups can be.
Humans cannot synthesize all the amino acids for proteins themselves. We have to consume protein to get these essential ingredients for survival. Here is one such amino acid, methionine. Can you identify each of its groups? (Note the R group has Sulfur)
Amino acids (monomers) combine in long chains to form a polypeptide (polymers). They do this by linking together at their acid and amino groups making long chains using peptide bonds.
Each long protein chain of amino acids will fold, twist and bend upon itself to form the final three-dimensional protein that serves one of many functions for living things. This final structure of the protein is what determines the function it will serve. The three-dimensional shape will determine if the protein will regulate cell processes, become a structural component, fight disease, or control the rate of chemical reactions. Take a look at the diagram below. In it, you can see how a chain of amino acids takes on its final shape:
Familiarize yourself with basic features of protein structure by clicking through the introductory tutorial from the University of Wisconsin.
Biological Catalysts: Enzymes
One of the most important types of proteins is called an enzyme. Enzyme characteristics:
- Enzymes act as biological catalysts (a catalyst is a chemical that speeds up chemical reactions within the cell).
- Enzymes are able to speed up the rate in which chemical reactions occur by reducing the activation energy (energy needed to start a chemical reaction).
- Enzymes work by providing a place for reactants to come together at a lower energy level so the products can come together faster.
- The reactants are called substrates and when they join together with the enzyme, an enzyme-substrate complex is formed; it is here that the substrates are converted into products and then released.
- Enzymes remain unchanged after the chemical reaction occurs and can be used over and over to complete the same reaction.
The Lactase Enzyme: An adaptation?
When we drink milk enzymes in our digestive system break down the lactose sugar in milk so that we can use the products for energy. The enzyme, lactase, fits with the lactose molecule and the bond between the two monosaccharides is then broken. The products are released and the enzyme is free to find another lactose molecule to act upon. Lactase will only act upon lactose. Its three-dimensional shape will not fit any other disaccharide.
Take a look at the picture on the right to see this process.
Previously, we discussed humans who are lactose intolerant. They do not make the enzyme to digest lactose, and therefore experience uncomfortable digestive symptoms when they consume dairy. Scientists speculate that lactase is an adaptation for a diet that became more dependent on dairy farming.
Enzymes are Specific for their Substrate
Enzymes can be synthetic (they can build up) or hydrolytic (they can break down, or digest). They are very specific for their substrates. When we name enzymes, convention is to use the suffix, -ase .
Here are some examples of other enzymes and their substrates:
|Lactase||Lactose||Glucose + Galactose|
|Lipase||Lipids||3 Fatty Acids + Glycerol|
Enzymes Require Optimal Conditions
Enzymes must have the best environmental conditions to operate most efficiently. This is called their optimum enzyme activity. The three conditions that limit enzyme activity are:
Temperature: Conditions can’t be too hot or too cold. Just like Goldilocks, the temperature has to be just right! Enzymes in your body operate best at/around 98.6 degrees Fahrenheit (23 degrees Celsius).
pH: Conditions can’t be too acidic or basic. They have to be just right. Remember that pH measures the hydrogen ion concentration in a solution (the more H+ the stronger the acid). Digestive stomach enzymes work best at a low pH. They are designed to work in an environment with lots of digestive stomach acid. Other parts of your body would have enzymes that work at a neutral pH.
Substrate and Enzyme concentration: The amount of each that is present must be-you guessed it-just right! There has to be sufficient amounts of both the enzyme and the substrate for the reaction to work.
What happens if an enzyme (or any protein) is exposed to an extreme pH or temperature? Denaturation occurs. The enzyme loses its three dimensional shape (and therefore its function). Denaturation is permanent and makes the enzyme non-functional.
As an example, take a look at the following pictures. The white of a chicken egg is made up of protein and water. When you fry an egg, and heat up this protein, you can see that it changes it’s physical nature – it becomes jelly-like and opaque. The proteins have been denatured.