While the traditional health food pyramid has really gone out the window, there is little argument that lean proteins are essential to a balanced diet.
While the traditional health food pyramid has really gone out the window, there is little argument that lean proteins are essential to a balanced diet. Proteins were almost crowned king of the biological molecules during the quest for heredity (1:06). Their diversity in structure is the greatest of all the biological molecules (2:35) with four levels as it bends and folds in upon itself (4:22). Proteins have a variety of functions, such as support, transport, recognition, movement, and communication (5:15).
The Question of the Day asks (8:02) How many stop codons are there?
Thank you for listening to The APsolute RecAP: Biology Edition!
(AP is a registered trademark of the College Board and is not affiliated with The APsolute RecAP. Copyright 2020 - The APsolute RecAP, LLC. All rights reserved.)
Website:
EMAIL:
Follow Us:
Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Proteins
Zoom out:
Unit 1 - Chemistry of life
Topic 1.4-1.5
Big idea - Systems interaction
Beef, Chicken, Eggs, Fish, Beans, Nuts, Soy - all excellent dietary sources of protein. While the traditional health food pyramid has really gone out the window, there is little argument that lean proteins are essential to a balanced diet. You are what you eat. So - let’s chow down! How do you make proteins? And why exactly are they so important? These biological molecules made of twenty amino acids are integrated into several units of biology - so a strong foundation is essential.
Let’s Zoom in:
Take a second to appreciate that several scientists wanted to crown proteins as the ruler of all biological molecules. It made sense - proteins have more “alphabet” in their chemistry than its counterpart DNA with 20 amino acids as compared to 4 nitrogenous bases. You can also form a lot more diverse structures for a multitude of functions with proteins than with DNA.
In an effort to determine the molecule of heredity, Griffith experimented with injecting varying bacteria into mice. His transforming principle got so far as to say that something from the heat-killed bacteria “transformed” the rough bacteria and made them lethal. Scientist Avery took this work a step farther to determine what the transforming molecule was. Through chemical tests observing the ratios of nitrogen to phosphorus, Avery showed that the chemical composition of the molecule matched DNA, not protein. Additionally, adding enzymes that break down DNA made the transforming principle inactive. In contrast, the addition of enzymes that break down proteins had no effect. Hershey and Chase then came onto the scene with radioactively tagged bacteriophages - protein with radioactive sulfur and DNA with radioactive phosphorus. Results showed that only the radioactive phosphorus had entered the bacteria, again confirming that DNA and not protein was the molecule of heredity.
Proteins are polymers (or polypeptides) of the monomer amino acids. Their diversity in structure is the greatest of all the biological molecules. An amino acid is made up of one central carbon atom that forms four single covalent bonds – one is to hydrogen, another to a carboxyl, and a third is to an amino group. The fourth covalent bond is to a variable “R” group. This R group is a substitute for different side chains that give each of the 20 different amino acids their unique properties (like size, charge, and pH). Amino acids join together through dehydration synthesis between an amino group of one monomer and the carboxyl group of the other. The covalent bond formed between them is referred to as a peptide bond.
How does a cell “know” what order to link the amino acids? Enter the Central Dogma! Genetic information flows from DNA to RNA to protein. The genetic code is first transcribed into mRNA which is translated into a sequence of amino acids. Translation occurs at ribosomes in both prokaryotic and eukaryotic cells. In short, rRNA reads three nucleotides of mRNA at a time, called codons. These codons correlate to one specific amino acid, which is brought over by tRNA. For example, if the DNA code reads TAC, then the complementary mRNA transcript is AUG (remember uracil is RNA only), and the corresponding amino acid is Methionine! This amino acid, among others, is then transferred to the growing polypeptide chain and the process is continued until a stop codon is reached. The newly synthesized polypeptide is released for further processing and shipping by the rough endoplasmic reticulum and golgi body in eukaryotes.
Proteins have four levels of structure as it bends and folds in upon itself. In fact, proteins are named as such, because Proteus of Greek mythology was a shape-shifter, able to take many forms! Primary structure is a chain of amino acids. Secondary can either form an alpha helix or a beta pleated sheet through local folding. Tertiary has a specific 3D shape when alpha helices and beta sheets fold further inwards. The final is quaternary, which has two or more separate amino acid chains, bent and folded and interacting together. The specific structure that each protein has is dependent upon the chemical properties of the R group and can be influenced by environmental factors. Ever notice how egg whites become opaque when fried and a curling iron alters hair shape? Protein structure denatures when hydrogen bonds are disrupted - this directly affects their function but can sometimes be reversed.
And these structures are very important and very specific to each purpose. One wrong amino acid and the protein folding will be incorrect, causing an error in function. The real question is - what CAN’T proteins do? They really seem to have their ‘R group” in everything. Here’s a few functions.
Proteins are a key component in cellular membranes, with roles in transport, recognition, movement, and communication. Integral membrane proteins will have hydrophobic regions that interact with phospholipid tails and hydrophilic regions that are adjacent to the heads. Some integral proteins have specific molecular chemistry internally, forming a channel. These channels may be gated and allow specific ions or small molecules to cross the membrane through facilitated diffusion or active transport. One of the most infamous integral carrier proteins is the sodium potassium pump. Peripheral proteins, like hormones and antigens, are more loosely attached to the membrane and are involved in cell recognition and communication. Membrane proteins can also provide anchorage for the cytoskeleton, aiding in structural support and cellular movement.
Proteins also form enzymes that catalyze chemical reactions. Enzymes are not used up in a reaction and are substrate and
condition specific. Why? Because structure influences function - and those R groups aren’t going to allow just any substrate to bind to the active site. For example, the enzyme sucrase has a specific active site which fits the substrate sucrose, a disaccharide. This enzyme facilitates the catabolic reaction of sucrose with water into the monomers glucose and fructose.
Time for unit connections. This episode mostly emphasizes Unit 1 with structure and function of proteins but they connect in so many other ways to the curriculum. You’ll hear about proteins in Unit 2 with organelle function and transport, Unit 3 with Enzymes, Unit 4 with signal transduction, Unit 6 with mutations, DNA replication, and central dogma, and Unit 7 with evidence of evolution. As for the exam? You might be presented with a scenario of a disrupted system and asked to make predictions (like transport, mutations or signaling), shown an enzyme activity graph, or maybe asked about R group interactions. Application is likely, so focus on understanding - not memorization! And if you need to use the codon chart, it will be provided.
To recap……
Proteins are polypeptides made of amino acids, structurally dictated by DNA and assembled during translation at a ribosome. By bending and folding through R group interactions, proteins have specific shapes and functions.
Coming up next on the Apsolute RecAP Biology Edition: Carbohydrates
Today’s question of the day is about translation
Question of the day: How many stop codons are there?