Problem 38 - General Chemistry: Principles and Modern Applications

Question

Write the structures of the different tetrapeptides that can be obtained from a combination of alanine, lysine, serine, and phenylalanine. Give the abbreviated formula of each (such as Ala-Lys-Ser-Phe), starting at the N-terminal end.

Step-by-Step Solution

Answer

The different tetrapeptides formed from Alanine, Lysine, Serine, Phenylalanine are: \[ Ala-Lys-Ser-Phe, Ala-Lys-Phe-Ser, Ala-Ser-Lys-Phe, Ala-Ser-Phe-Lys, Ala-Phe-Ser-Lys, Ala-Phe-Lys-Ser, \] \[Lys-Ala-Ser-Phe, Lys-Ala-Phe-Ser, Lys-Ser-Ala-Phe, Lys-Ser-Phe-Ala, Lys-Phe-Ser-Ala, Lys-Phe-Ala-Ser, \] \[Ser-Ala-Lys-Phe, Ser-Ala-Phe-Lys, Ser-Lys-Ala-Phe, Ser-Lys-Phe-Ala, Ser-Phe-Lys-Ala, Ser-Phe-Ala-Lys, \] \[Phe-Ala-Lys-Ser, Phe-Ala-Ser-Lys, Phe-Lys-Ala-Ser, Phe-Lys-Ser-Ala, Phe-Ser-Lys-Ala, Phe-Ser-Ala-Lys \]

Step 1: Understand the concept of tetrapeptides

Tetrapeptides are peptides that are formed by the combination of four distinct amino acids. In this exercise, the four distinct amino acids are Alanine (Ala), Lysine (Lys), Serine (Ser), and Phenylalanine (Phe). Each combination will form a unique tetrapeptide.

Step 2: Find all possible combinations

In order to find all possible combinations of the tetrapeptides, we must consider each and every position for each amino acid. Any of the four amino acids can be the first in the chain, any remaining amino acid can be second, and so on. So, the total number of possible combinations is 4! (4 factorial) which equals to 24 possible tetrapeptides.

Step 3: Construct the peptide sequences

Start writing all possible combinations of these four amino acids in a sequence of four, starting from the N-terminal end. Some of the combinations are: Ala-Lys-Ser-Phe, Ala-Lys-Phe-Ser, Ala-Ser-Lys-Phe, etc. Continue this process until all 24 combinations are formed.

Key Concepts

Amino Acid Combination

Amino acids are the building blocks of proteins and play a central role in biochemistry. In the context of tetrapeptides, we are focusing on the combination of four specific amino acids to form a peptide chain.

Each amino acid has its unique structure characterized by a central carbon atom, called the alpha carbon, an amino group, a carboxyl group, and a distinctive side chain, known as the R group. The combination of these amino acids in peptide chains is not random; it's dictated by the peptide sequence which is ultimately determined by the genetic code.

When combining amino acids like alanine, lysine, serine, and phenylalanine to form tetrapeptides, it's essential to understand that each order represents a different molecule with potentially different properties and functions. The variety of possible tetrapeptide combinations from just these four amino acids showcases the incredible diversity of molecular structures possible in protein chemistry.

Peptide Bond Formation

Peptide bonds are the linkages that hold amino acids together in a peptide chain. They are a type of covalent bond specifically known as an amide bond.

The process of forming a peptide bond involves a dehydration synthesis reaction, where the amino group of one amino acid reacts with the carboxyl group of another, releasing a molecule of water and creating the bond. This bond forms between the carbon of the carboxyl group and the nitrogen of the amino group.

In our tetrapeptide scenario, every step of growth for the peptide chain involves the formation of a new peptide bond, meticulously joining one amino acid after another in a predetermined sequence. The formation is catalyzed by enzymes in biological systems but can also occur under suitable conditions in a lab setting.

Peptide Sequence Notation

Peptide sequence notation is a way to symbolize the order of amino acids in a peptide. It provides a shorthand method to convey a potentially complex peptide structure.

The notation starts at the N-terminal end, which is the end of the peptide with a free amino group, and ends at the C-terminal, where the free carboxyl group resides. Each amino acid is typically represented by either a three-letter abbreviation, such as Ala for alanine or a single-letter code, A in the case of alanine.

When writing out the sequence for tetrapeptides, like Ala-Lys-Ser-Phe, each abbreviation is separated by a hyphen to signify the peptide bonds between them. The sequence direction is critical, as peptide chains are directional, and the function of peptides can dramatically change with alteration in sequence.

Protein Structure

Protein structure is an intriguing aspect of biochemistry, as the specific three-dimensional shape of a protein dictates its function. Proteins can have four levels of structure - primary, secondary, tertiary, and quaternary.

The primary structure is the linear sequence of amino acids, linked by peptide bonds, which we explore when we write the sequence of a tetrapeptide. This sequence determines how the protein will fold into its secondary structure - common motifs include alpha helices and beta pleated sheets. These, in turn, fold further into a tertiary structure, a unique 3D shape that can be globular or fibrous. If the protein consists of multiple peptide subunits, their arrangement in space is referred to as its quaternary structure.

Every level of structure is crucial because a change as small as a single amino acid can alter the protein’s structure and, consequently, its function - reflecting the importance of accuracy in peptide sequence notation and understanding the implications of amino acid combination.