Problem 46 - Chemistry Central Science

Question

Proteins are polymers formed by condensation reactions of amino acids, which have the general structure

Step-by-Step Solution

Answer

The structure of alanine is obtained by replacing the R group in the general amino acid structure, H₂N-CH(R)-COOH, with a methyl group (CH₃). Thus, the structure of alanine is H₂N-CH(CH₃)-COOH, or in a drawn format: \[ H_{2}N - \underset{\displaystyle |}{\overset{\displaystyle H}{\overset{\displaystyle |}{\underset{\displaystyle CH_{3}}{C}}}} - COOH \]

Step 1: Recall the general structure of an amino acid

An amino acid has the general structure H₂N-CH(R)-COOH, where R is the side chain specific to each amino acid.

Step 2: Identify the side chain of alanine

The exercise mentions that alanine has a methyl side chain (CH₃). Therefore, the R group in the general structure of the amino acid must be replaced with a methyl group.

Step 3: Draw the structure of alanine

Replace the R group with CH₃ in the general structure of an amino acid: H₂N-CH(CH₃)-COOH. Hence, the structure of alanine is as follows: \[ H_{2}N - \underset{\displaystyle |}{\overset{\displaystyle H}{\overset{\displaystyle |}{\underset{\displaystyle CH_{3}}{C}}}} - COOH \] The structural formula above represents alanine with a methyl side chain.

Key Concepts

Understanding Amino Acids

Amino acids are often described as the building blocks of proteins. They play a crucial role in various biological processes, including the synthesis of proteins and neurotransmitters. Each amino acid molecule is composed of a central carbon atom, also referred to as the \( \alpha \) carbon, which is bonded to four different groups: an amino group (\( -NH_2 \)), a carboxyl group (\( -COOH \)), a hydrogen atom, and a distinctive side chain, represented by \( R \) in general formulas.

There are 20 amino acids that are commonly found in proteins, each with a unique side chain that determines its characteristics and function. For instance, the side chain might make the amino acid polar or nonpolar, acidic or basic. This \( R \) group distinction is what gives each amino acid its unique properties and influences how it interacts with other amino acids within a protein molecule.

When students encounter problems related to amino acids, it can be helpful to break down the molecule's structure systematically, identifying each component and understanding how they come together to form the whole amino acid.

Condensation Reactions Explained

Condensation reactions are a type of chemical reaction where two molecules combine to form a larger molecule, with the loss of a small molecule, often water. This process is fundamental in the creation of polymers, and it's precisely how amino acids link together to form proteins. In the case of amino acids, the amino group of one amino acid bonds to the carboxyl group of another, releasing water in the process and forming a peptide bond.

In biochemistry, this reaction is also known as a dehydration synthesis because it synthesizes a larger molecule while dehydrating the reactants. It's important for students to recognize that this is the key mechanism by which the long chains of proteins are constructed, with the resulting polymer being a series of amino acids connected by peptide bonds. Understanding condensation reactions provides insight into the structural formation of a variety of biological macromolecules beyond just proteins.

Molecular Structure of Alanine

Alanine is one of the twenty standard amino acids used in protein synthesis. It is classified as a nonpolar, aliphatic amino acid. Based on its structural simplicity, alanine serves as a great introductory example when studying amino acid structures. The molecular structure of alanine includes the central \( \alpha \) carbon bonded to a methyl group (\( -CH_3 \)) as its side chain or \( R \) group.

To visualize alanine, one can draw the general amino acid backbone, with the \( \alpha \) carbon at the center, attached to an amino group on one side and a carboxyl group on the other. The \( \alpha \) carbon is also bonded to a single hydrogen atom and the methyl group. The molecular formula can be represented as \(H_{2}N-CH(CH_{3})-COOH\), clearly showing the methyl group as the defining characteristic of alanine. By understanding the structure of alanine, students can begin to appreciate how varying side chains impact the overall properties and roles of different amino acids in proteins.