What is a protein? Intro to Biochemistry

My PhD is in macromolecular and structural biochemistry. More specifically I study the dynamics of proteins using X-ray crystallography. Before I can start any conversation about my research, I first ask, "do you know what a protein is?" It helps me gauge the level of understanding of the person I'm conversing with.

The central dogma of biochemistry is... DNA -> mRNA -> Protein.

Most people know DNA as the language that encodes genetic information. The instructions for each gene are transcribed into messenger RNA (mRNA), and a small organelle called the ribosome translates the RNA message into protein.

Humans have roughly 19,000 distinct genes. One gene often encodes multiple splice variations, so humans are estimated to have 30,000 - 35,000 different expressed proteins, and each one has a different role to perform within the cell.

The concentration of protein in a typical cell is 200 g/l, which is roughly 20% of the cell volume, and water fluctuates between 70-80%. This means the majority of the cellular milieu is protein. It is pretty amazing how all the proteins in the cell are able to communicate synchronously with each other despite all the traffic http://www.mcponline.org/content/13/12/3497.full

http://iopscience.iop.org/article/10.1088/1478-3975/2/2/S01/fulltext/

Proteins are a polymer of amino acids. The sequence of amino acids is called the primary structure. If you could imagine, each amino acid is a link a chain. Although the length of these chains can vary from 2 to 3000+ amino acids, a protein is typically considered longer than 50 amino acids. Aspartame is actually a dimer of two amino acids (aspartate and phenylalanine).

Each of the 20 amino acids have different physical characteristics, which causes the chain of amino acids to adopt a canonical structure, called the secondary structure (depicted as helices and arrows in the image below). The structure of a protein determines its function, and the structure is evolutionarily conserved. Mutations that affect a protein's structure are often the basis for prevalent diseases and disorders. For example, sickle cell anemia is cause by replacing one link the amino acid chain of hemoglobin. https://evolution.berkeley.edu/evolibrary/article/mutations_06

this is a model of the caspase-6 protein (made with X-ray crystallography), and only shows the secondary and tertiary structure.

So, when we eat "protein" we are essentially recycling the amino acids from an external source, so that we can re-use the amino acid building blocks for our own cells.

The take home point is that "protein" does not translate to "meat."

-The Scientist