3.6
Proteins and Amino Acids
Proteins are chains of amino acids that are connected by peptide bonds and folded into a 3-dimensional structure. The side chains of individual amino…
A protein is a chain of amino acids. At physiological pH, amino acids exist predominantly in a zwitterionic form, carrying both, a positive and a negative charge. Each amino acid contains amine and carboxyl groups, along with a different side group, denoted as R, attached to a core carbon.
Amino acids are linked through covalent peptide bonds, and the repeating series of atoms along the core is called the polypeptide backbone.
The sequence begins at the N-terminus, with a free amino group, NH3+, and ends with the C-terminus, a free carboxyl group, COO-.
Each protein is unique, depending on the number of amino acids and their side chains. For example, thyroid-releasing hormone has 3 amino acids, whereas the elastic muscle protein titin has over 34,000.
There are 20 standard amino acids, each with a different side chain, and they fall into one of four groups.
Negative polar amino acids, such as aspartic and glutamic acid, have side chains with a carboxyl group. Because of this, they carry a negative charge at physiological pH.
Positive polar amino acids carry a positive charge at physiological pH and have nitrogen-containing side chains.
Several amino acids are polar but uncharged. They are usually hydrophilic and form the outer portion of a folded protein, where they can interact with the aqueous environment.
Lastly, a large group of amino acids has nonpolar side chains.
They can be simple, as in glycine, or complex, like proline and tryptophan.
Nonpolar side chains generally make the amino acid hydrophobic, forming the core of many proteins.
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Q1: What is the basic structure of an amino acid?
An amino acid contains a carboxyl group and an amino group attached to the same carbon atom, called the alpha-carbon. A side chain, or R-group, extends from this central carbon and determines the amino acid's chemical identity and properties. The simplest amino acid is glycine, which has only a hydrogen atom as its side chain.
Q2: How are amino acids linked together in proteins?
Amino acids are linked through covalent peptide bonds formed between the amino group of one amino acid and the carboxyl group of an adjacent amino acid. This process occurs through dehydration synthesis, where one water molecule is released during each linkage. The resulting chain forms the polypeptide backbone, with a free amino group at the N-terminus and a free carboxyl group at the C-terminus.
Q3: What are the four main groups of amino acid side chains?
The 20 amino acids used in proteins have side chains classified into four groups: negative polar amino acids with carboxyl groups that act as acids; positive polar amino acids with amino groups that act as bases; polar uncharged amino acids that are hydrophilic; and nonpolar amino acids with hydrophobic side chains. These properties determine how amino acids interact and fold within proteins.
Q4: How does pH affect the chemical properties of amino acids?
At low pH, both carboxyl and amino groups are protonated, making the amino acid act as a base. At high pH, both groups are deprotonated, causing it to act as an acid. At neutral pH, typical in physiological environments, the amino group is protonated and the carboxyl group is deprotonated, creating a zwitterion with both positive and negative charges.
Q5: What determines the uniqueness of each protein?
Each protein's uniqueness depends on the number of amino acids in its chain and the specific sequence of their side chains. For example, thyroid-releasing hormone contains 234 amino acids, while the elastic muscle protein connectin has over 34,000. The side chain composition and arrangement directly influence how the protein folds and functions.
Q6: What is the difference between a polypeptide and a protein?
A polypeptide is a chain of amino acids linked by peptide bonds, with chains containing fewer than 20 amino acids called oligopeptides. A protein is a polypeptide that has folded into a three-dimensional structure and is ready to perform its specific cellular function. Length, structural complexity, and functionality distinguish proteins from simpler polypeptide chains.
Q7: What are the major functions of proteins in cells?
Proteins serve diverse cellular functions including providing structural support like collagen, enabling movement through actin and myosin in muscles, catalyzing reactions as enzymes, transporting molecules across cell membranes, and defending organisms through antibodies. Together with carbohydrates, nucleic acids, and lipids, proteins are fundamental building blocks essential to all life.