Question

Choose one of the following terms to match the description given. a. aldohexose n. glycogen b. saliva o. glycoside linkage c. antibody p. hormone d. cellulose q. hydrophobic e. \(\mathrm{CH}_{2} \mathrm{O}\) I. inhibition f. cysteine s. ketohexoses g. denaturation t. oxytocin h. disaccharides u. pleated sheet i. disulfide v. polypeptide j. DNA w. polysaccharides k. enzymes x. primary structure 1\. fibrous y. substrate m. globular z. sucrose linkage that forms between two cysteine species

Short answer

The correct answer is option 'i' (disulfide).

Step-by-step solution

Identify the terms related to cysteine

Cysteine itself is present in the list as option 'f'. We can also notice the term 'disulfide' as option 'i'. Since the description asks for the linkage between cysteine species, we should focus on option 'i'.

Confirm the correct answer

The term 'disulfide' in option 'i' represents the linkage that forms between two cysteine species, which is called a disulfide bond or disulfide bridge. Therefore, the correct answer for the description is option 'i' (disulfide).

Key concepts

Cysteine

Cysteine is a unique amino acid important in biochemistry. It is one of the 20 standard amino acids used to build proteins in the human body. This amino acid is particularly known for its sulfur-containing side chain, called a thiol group (\(-SH\)). The sulfur atom in cysteine allows it to form special bonds, making it quite distinct among other amino acids.
Characteristics of cysteine include:

• Polar nature: Cysteine is somewhat polar due to its thiol group, allowing it to interact with water and other molecules.
• Reactive thiol group: The thiol group is highly reactive, enabling cysteine to form disulfide bonds, which are crucial for protein structure.
• Role in metabolism: Cysteine is synthesized in the human body using serine, another amino acid, and contributes to various metabolic processes.

Cysteine plays a significant role in maintaining protein structure by forming disulfide bonds that stabilize proteins, particularly those secreted outside of cells.

Disulfide bond

A disulfide bond, also known as a disulfide bridge, is a covalent linkage vital for structural integrity in proteins. This bond forms between the sulfur atoms of two cysteine molecules, creating a link that can significantly alter the protein's shape and stability.
Key features of disulfide bonds include:

• Formation: Disulfide bonds form during the protein folding process when two cysteine residues come into contact and their thiol groups oxidize to form a \(\text{-S-S-}\) bond.
• Stability: The bond is much stronger than many other types of interactions in proteins, providing extra structural stability, especially in extracellular proteins.
• Function: Disulfide bonds help maintain a protein's functional form, or its tertiary and quaternary structures. They play a pivotal role in processes like enzyme activity and immune response.

Disulfide bonds are reversible and can break, leading to changes in the protein's structure and function, often through reduction reactions.

Amino acids

Amino acids are the building blocks of proteins, fundamental to various biological processes in living organisms. There are 20 standard amino acids, each contributing unique properties to the proteins or enzymes they form.

• Basic structure: Amino acids have a central carbon (\(\alpha\)-carbon) to which an amino group (\(-NH_2\)), a carboxyl group (\(-COOH\)), a hydrogen atom, and a distinct side chain (R group) are attached.
• Side chain variety: The R group determines an amino acid's characteristics, such as polarity, charge, and reactivity.
• Categories: Amino acids can be essential (obtained through diet) or non-essential (synthesized by the body).

Each amino acid sequence and interaction determine the final three-dimensional shape and function of proteins, making amino acids crucial for life.

Protein structure

Protein structure is fundamental to understanding how proteins perform their diverse roles in organisms. The structure of a protein is generally categorized into four levels:

• Primary structure: This is the sequence of amino acids in a polypeptide chain, determined by genetic information. It dictates all higher levels of protein structure.
• Secondary structure: Hydrogen bonds between backbone atoms lead to formations like alpha helices and beta-pleated sheets, adding folds to the protein.
• Tertiary structure: This level involves 3D folding due to interactions between R groups, including hydrogen bonds, disulfide bridges, ionic bonds, and hydrophobic interactions, providing functional form.
• Quaternary structure: Some proteins consist of multiple polypeptide chains, and their arrangement constitutes the quaternary structure.

Understanding protein structure is crucial for insights into their functions and how alterations might lead to diseases. This level of understanding informs the development of pharmaceuticals, treatments, and diagnostics.