Question

Consider the following statements about proteins: 1\. all natural amino acids which are constituents of proteins are \(\alpha\)-amino acids 2\. \(\alpha\)-amino acids are all optically active and have the L-configuration 3\. an especially favourable conformation for the peptide linkage in proteins is the \(\alpha\)-helix arrangement. 4\. \(\alpha\)-amino acids are connected by ester linkages of these statements (a) 1 and 3 are correct (b) 1 and 2 are correct (c) 2 and 3 are correct (d) 2,3 and 4 are correct

Short answer

The correct statements are 1 and 3, so (a) is correct.

Step-by-step solution

Analyze Statement 1

Statement 1 asserts that all natural amino acids in proteins are \( \alpha \)-amino acids. This is correct because in proteins, amino acids are indeed \( \alpha \)-amino acids, which means the amino group is attached to the carbon next to the carboxyl group.

Analyze Statement 2

Statement 2 claims that all \( \alpha \)-amino acids are optically active and have the L-configuration. However, this statement overlooks glycine, which is not optically active as it has two hydrogen atoms attached to the \( \alpha \)-carbon, making it achiral.

Analyze Statement 3

Statement 3 describes the \( \alpha \)-helix as a favorable conformation for the peptide linkage in proteins. This is correct, as the \( \alpha \)-helix is indeed one of the most common and stable structures in the secondary structure of proteins.

Analyze Statement 4

Statement 4 says \( \alpha \)-amino acids are connected by ester linkages. This is incorrect because amino acids in proteins are connected through peptide bonds, not ester linkages.

Determine Correct Statements

From the analyses: Statements 1 and 3 are correct, while 2 and 4 are incorrect.

Key concepts

Amino Acids

Amino acids are the building blocks of proteins. Each amino acid has a basic structure consisting of an amino group \((-NH_2)\), a carboxyl group \((-COOH)\), a hydrogen atom, and a distinctive side chain or R-group, all attached to a central carbon atom called the \(\alpha\)-carbon. Protein-forming amino acids are typically referred to as \(\alpha\)-amino acids because the amino group is attached to the \(\alpha\)-carbon. Each amino acid is unique due to its side chain, which can vary in size, polarity, and charge. This diversity allows proteins to have a vast array of functions. There are 20 different \(\alpha\)-amino acids that are commonly found in proteins, each with the same basic structure but differing in the nature of their side chains.

Peptide Linkage

Peptide linkages, or peptide bonds, are the connections between amino acids in a protein chain. These bonds form through a dehydration synthesis reaction where the carboxyl group of one amino acid reacts with the amino group of another, releasing a molecule of water \(H_2O\). This results in a covalent bond known as a peptide bond \(C-N\), linking the amino acids. The chain of amino acids created by these linkages is called a polypeptide, which then folds into a specific three-dimensional structure to become a functional protein. Peptide bonds are strong and provide the protein molecule with stability.

Optical Activity

Optical activity refers to the ability of a compound to rotate plane-polarized light, a characteristic of chiral molecules. Most \(\alpha\)-amino acids are chiral and exist in two enantiomers, the L- and D-forms. In biological systems, only L-amino acids are typically found. However, there are exceptions, such as glycine, which is achiral because its \(\alpha\)-carbon is attached to two hydrogen atoms, giving it no optical activity. Optical activity in proteins is crucial for their biological function, as it allows them to interact specifically with other chiral molecules.

Alpha Helix

The \(\alpha\)-helix is a key element of the protein secondary structure. This conformation is characterized by a right-handed coil where each backbone \(C=O\) group forms a hydrogen bond with an \(N-H\) group four residues ahead in the sequence. This stable arrangement gives the \(\alpha\)-helix its strength and flexibility, allowing it to be a common structural motif in proteins. The \(\alpha\)-helix helps in compact folding and provides flexibility to the protein structure, contributing significantly to the protein's overall functional form.