Proteins are organic biological molecules, classified as biomolecules, and are essential components of living cells.
They always contain carbon, hydrogen, oxygen, and nitrogen. Without these four elements, proteins cannot form.
Some proteins may also contain small amounts of sulfur, phosphorus, iron, iodine, or magnesium, but these are not always present.
Unlike carbohydrates and lipids, which only contain carbon, hydrogen, and oxygen, proteins also include nitrogen.
Proteins are the most abundant biomolecules in cells, making up about 50% of the cellтАЩs dry weight. In a typical cell, water makes up 70%, and proteins account for a significant portion of the remaining 30% dry matter.
Proteins are also called polypeptides because they are made up of long chains of amino acids linked by peptide bonds.
Introduction and Structure of Amino Acids
Proteins are always composed of amino acids, which are joined together by peptide bonds.
Amino acids are the building blocks of proteins, similar to how monosaccharides build carbohydrates and fatty acids plus glycerol build lipids.
There are many amino acids found in nature, but only 20 are commonly found in proteins and are most important for animals, including humans.
Each amino acid has a central (alpha) carbon atom attached to:
A hydrogen atom
An amino group (NH2)
A carboxyl group (COOH)
A variable side chain called the R group
The R group is different in each of the 20 amino acids and determines the unique chemical properties and characteristics of each amino acid.
The general structure of an amino acid can be represented as:
Alpha carbon (C) bonded to H, NH2, COOH, and R group.
All 20 amino acids share this basic structure, with differences only in the R group.
Types of Amino Acids
Amino acids are categorized into two main groups: essential and non-essential.
Essential Amino Acids:
These cannot be synthesized by the human body and must be obtained from the diet.
They are found in both animal sources (milk, eggs, fish, chicken, meat) and plant sources (vegetables, lentils).
Essential for muscle and bone repair, hormone synthesis, and neurotransmitter production.
Deficiency can lead to various health problems.
There are 9 essential amino acids: lysine, leucine, isoleucine, histidine, methionine, phenylalanine, threonine, valine, and tryptophan.
Non-Essential Amino Acids:
The body can synthesize these amino acids, usually by modifying other amino acids or food components.
Deficiency is rare, but may occur in cases of illness or starvation.
Non-essential amino acids play roles in detoxification in the liver, production of red and white blood cells, nerve impulse transmission, and heart function.
There are 11 non-essential amino acids: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.
A pattern can be noticed in the names of non-essential amino acids, as several start with A, G, C, S, or P, which can help in memorization.
Key Terms & Definitions
Protein: Large, complex molecules made up of long chains of amino acids; essential organic biomolecules in living organisms.
Polypeptide: Another term for protein; refers to a chain of amino acids linked by peptide bonds.
Amino Acid: The basic building block of proteins, each containing an alpha carbon, hydrogen, amino group (NH2), carboxyl group (COOH), and a variable R group.
Essential Amino Acids: Amino acids that cannot be produced by the body and must be obtained from dietary sources.
Non-Essential Amino Acids: Amino acids that the body can synthesize on its own, so they are not required in the diet.
Action Items / Next Steps
Memorize the names and classification (essential vs. non-essential) of the 20 amino acids.
Practice drawing the general structure of an amino acid, showing the alpha carbon, NH2, COOH, H, and R group.
Prepare to study the specific structures and properties of glycine and alanine in the next lecture.
Try to find or create mnemonics or patterns to help remember the lists of essential and non-essential amino acids.