We have looked at lipids and now it is time to focus on proteins. I love proteins! Although proteins are most often associated with meat, proteins are part of many other foods as well. They do so many cool things. Bromelain, found in pineapples, is a meat tenderizer. Gluten, in wheats and other grains, makes dough elastic. Myoglobin carries oxygen within the muscle.
Of the three macro nutrients, I think proteins are the most complex. They are huge, compared to other molecules, and have many different purposes. But that is what makes them so fun to study.
The Building Blocks
Proteins are made of amino acids. Amino acids all have the same base: a carboxyl group, an amine group, a hydrogen, and an R group. Remember carboxyl group from lipids? It has two oxygens bonded to a carbon. The amine group has a nitrogen. These two groups are where amino acids link together through a peptide bond. The bond is formed when one amino acid gives up a hydrogen from its amine group and the other amino acid looses a hydrogen and oxygen from its carboxyl group to form water (H2O).
The R group is the only thing that changes from one amino acid to the next. For some amino acids, the R group is just a few extra atoms. Other amino acids have very large R groups.
Four Types of Structure
Did I mention that proteins are complex? Unlike triglycerides that have one basic form (fatty acids attached to a glycerol), proteins come in all shapes and sizes. In the world of molecules, proteins are huge. There can be well over one hundred amino acids in a chain! These chains bend, twist, and fold into complicated masses. There is method within the madness though. There are four different types of structure within a protein.
Primary structure: the order in which the amino acids are connected together. Just like letters make up different words based on which letters are used and how they are arranged, amino acids make different proteins based on their primary structure.
Secondary structure: whether a chain of amino acids curls into an alpha helix or zigzags into a beta sheet. The chain does not stay straight because the R groups that are close to each other participate in hydrogen bonding.
Tertiary structure: the overall 3D arrangement. It is a result of interactions between R groups on amino acids that are further apart. For example, the sulfur in cysteine can form covalent disulfide bonds. This is where amino acids start looking pretty crazy.
Quaternary structure: how multiple amino acid chains fit together. Not all proteins have more than one chain, but some do.
Why care about protein structure?
The structure of a protein impacts what the protein does. Within the corkscrews, bends, and folds of a protein some sections are hidden, while other sections are able to interact with their surroundings. The interactions are what allow proteins to do what they do.
Enzymes are a specific type of protein that are catalysts, which means they make chemical reactions occur faster and/or more readily. The enzyme bromelain, from pineapples, tenderizes meats by catalyzing the break down of the protein collagen. Enzymes have active sites, places within their structure that can interact with their surroundings. Typically these active sites are ideal pockets for specific molecules. Bromelain has an active site where it attaches to collagen. Active sites are called active for a reason, they are where the action happens. Once collagen is in bromelain’s active site a series of chemical reactions occur both in bromelain’s and collagen’s structure. The end result is the break down of collagen and bromelain returns to its original structure.
When a protein doesn’t maintain its structure, it is said to be denatured. Denatured proteins can’t do what they are supposed to do. A denatured bromelain will not break down collagen because the some areas along its protein chain that were hidden from their surroundings are exposed and the exposed areas might be hidden. This changes the active site and the chemical reactions that can occur.
Sometimes we want the proteins to denature. For example, albumen turns an egg white from clear to white and from slime to gel when it is denatured. Many proteins denature when exposed to high heat, extreme acidic environments, extreme basic environments, or high mechanical stress.
So that is protein structure in a nut shell. Proteins are made from amino acids. There are four levels of protein structure which impact the function of a protein. When a protein does not hold its structure it is denatured.
Next up in the macro nutrient series: carbohydrates.
Images: 3D models created with Jmol; Jason Koval & Kevin Cartwright; P.E. Stein, A.G. Leslie, J.T. Finch, R.W. Carrell & RCSB PDB; Richard Wheeler & RCSB PDB
Source: Principles of Biochemistry, 5th Edition, by David L. Nelson and Michael M. Cox