Question

Draw a structural formula for Lys-Phe-Ala. Label the \(N\)-terminal amino acid and the \(C\)-terminal amino acid. What is the net charge on this tripeptide at \(\mathrm{pH} 6.0\) ?

Short answer

Question: Draw the structural formula of the tripeptide Lys-Phe-Ala, label the N-terminal and C-terminal amino acids, and determine their net charge at pH 6.0. Answer: The structural formula of the tripeptide Lys-Phe-Ala is \(\ce{H3N+-CH(CO2)-CH2-CH2-CH2-CH2-NH-CH(CO2)-CH2-C6H5-CH(NH2)-CO2-}\), with Lysine as the N-terminal and Alanine as the C-terminal amino acids. The net charge of Lys-Phe-Ala at pH 6.0 is +1.

Step-by-step solution

Amino Acid Structures

First, let's remember the structure of the three amino acids. Lysine (Lys) is a basic amino acid with a positive side-chain, Phenylalanine (Phe) is a hydrophobic amino acid with a nonpolar side chain, and Alanine (Ala) is also a hydrophobic amino acid with a nonpolar side chain: 1. Lysine: \(\ce{NH2-CH(CO2H)-CH2-CH2-CH2-CH2-NH2}\) 2. Phenylalanine: \(\ce{NH2-CH(CO2H)-CH2-C6H5}\) 3. Alanine: \(\ce{NH2-CH(CO2H)-CH3}\)

Connecting the Amino Acids

Now, let's connect each amino acid by forming a peptide bond through a condensation reaction between the amino group of one amino acid and the carboxyl group of the next amino acid: 1. Peptide bond between Lysine (\(\ce{N}\)-terminal) and Phenylalanine: Lysine's amino group (\(\ce{NH2}\)) will react with Phenylalanine's carboxyl group (\(\ce{CO2H}\)) and form a peptide bond, releasing a water molecule. 2. Peptide bond between Phenylalanine and Alanine (\(\ce{C}\)-terminal): Phenylalanine's amino group (\(\ce{NH2}\)) will react with Alanine's carboxyl group (\(\ce{CO2H}\)) and form a peptide bond, releasing a water molecule. After forming the peptide bonds, our tripeptide Lys-Phe-Ala looks like this: \(\ce{H3N+-CH(CO2)-CH2-CH2-CH2-CH2-NH-CH(CO2)-CH2-C6H5-CH(NH2)-CO2-}\)

Determining the Net Charge

Now, let's determine the net charge on Lys-Phe-Ala at pH 6.0. We'll consider the ionizable groups: amino and carboxyl groups on N-terminal (Lys) and C-terminal (Ala) and the side-chain of Lysine. Their pKa values are as follows: 1. N-terminal amino group (Lysine): pKa = 9.0 2. C-terminal carboxyl group (Alanine): pKa = 2.0 3. Lysine side-chain amino group: pKa = 10.5 Comparing the pKa values to the given pH 6.0, we can determine the charge of each group: 1. N-terminal amino group (Lysine): pH 6.0 < pKa 9.0; the group will be protonated, and carry a positive charge. 2. C-terminal carboxyl group (Alanine): pH 6.0 > pKa 2.0; the group will be deprotonated, and carry a negative charge. 3. Lysine side-chain amino group: pH 6.0 < pKa 10.5; the group will be protonated, and carry a positive charge. Now let's sum the charges: Positive charge: \(+1\) from N-terminal amino group (Lys) and \(+1\) from Lysine side-chain amino group. Negative charge: \(-1\) from C-terminal carboxyl group (Ala). Net charge on Lys-Phe-Ala at pH 6.0: \((+1) + (+1) + (-1) = +1\)

Key concepts

Peptide Bond Formation

At the heart of proteins and peptides lies the critical chemical bond known as the peptide bond. This bond forms when two amino acids, the building blocks of proteins, are linked together. This occurs during a condensation reaction where the carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH2) of another, releasing a water molecule and resulting in a dipeptide.

The new bond formed is a covalent bond between the carbon atom of the carboxyl group and the nitrogen atom of the amino group, specifically termed a peptide bond or an amide bond. This reaction is repeated over and over, creating long chains of amino acids, known as polypeptides, which can fold into complex three-dimensional structures to form proteins.

Understanding peptide bond formation is crucial for comprehending protein structure and function. It's also essential for learning biochemistry and molecular biology, where the manipulation of peptide bonds can lead to significant advances in drug development and biotechnology.

Amino Acid Structure and Properties

Amino acids are fascinating molecules that serve as the monomers for proteins. Each consists of a central carbon atom (the alpha carbon) bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a distinctive side chain (R group). It's the unique R group that determines the properties and category of the amino acid – whether it's basic, acidic, polar, nonpolar, or aromatic.

For example, lysine is known for having a positively charged side chain at physiological pH, making it a basic amino acid. Alanine, with its simple methyl group as an R group, is nonpolar, while phenylalanine features a bulky benzyl side chain, classifying it as an aromatic and hydrophobic amino acid. These side chains play a pivotal role in how amino acids interact with each other and with other molecules, and ultimately, they influence the structure and function of the produced polypeptides and proteins.

Additionally, the amino and carboxyl groups can gain or lose protons (H+ ions) depending on the pH of the surrounding environment, which affects their charge. Understanding the properties of amino acids is vital for predicting the behavior of peptides and proteins in different environments. Amino acid structure and properties tie into fields like nutrition, disease treatment, and enzyme function.

Determining Peptide Net Charge

The overall charge of a peptide at a given pH is the sum of the charges of its individual amino acids. The charge state of amino acids is influenced by the pH of the environment and each amino acid's pKa values, the pH at which the amino acid has an equal number of positive and negative charges.

An amino acid will tend to be positively charged if the pH of the solution is below its pKa, as it will have protonated amino groups (NH3+). Conversely, it will be negatively charged if the pH is above its pKa, due to deprotonated carboxyl groups (COO-).

When calculating the net charge of a peptide at a specific pH, it's crucial to account for the pKa values of the terminal amino (N-terminal) and carboxyl (C-terminal) groups, as well as any ionizable side chains. For instance, in the exercise, lysine's side chain has a pKa of 10.5, which remains protonated and thus positively charged at pH 6.0. Understanding how to determine peptide net charge aids in techniques such as isoelectric focusing and can help predict how peptides will interact in biological systems.