Question

Chaperone proteins A. all require ATP to exert their effect. B. cleave incorrect disulfide bonds, allowing correct ones to subsequently form. C. guide the folding of polypeptide chains into patterns that would be thermodynamically unstable without the presence of chaperones. D. of the Hsp70 class are involved in transport of proteins across mitochondrial and endoplasmic reticulum membranes. E. act only on fully synthesized polypeptide chains.

Short answer

Answer: Of the Hsp70 class are involved in transport of proteins across mitochondrial and endoplasmic reticulum membranes.

Step-by-step solution

Understanding Chaperone Proteins

Chaperone proteins are molecular machines that assist other proteins in their folding process. These proteins play an important role in cellular processes, making sure that proteins fold correctly and function properly. Let's evaluate each statement relating to chaperone proteins to determine which one is accurate.

Evaluating the Statements

A: All require ATP to exert their effect. Not all chaperone proteins require ATP to function. Although some do use ATP to assist in protein folding, others do not rely on ATP for their function. B: Cleave incorrect disulfide bonds, allowing correct ones to subsequently form. This statement is inaccurate. Chaperone proteins do not cleave disulfide bonds; their primary function is to facilitate protein folding. C: Guide the folding of polypeptide chains into patterns that would be thermodynamically unstable without the presence of chaperones. Chaperone proteins assist in the folding process, stabilizing intermediate structures and preventing aggregation. However, the final folded structure of the proteins is thermodynamically stable. Hence, this statement is inaccurate. D: Of the Hsp70 class are involved in transport of proteins across mitochondrial and endoplasmic reticulum membranes. Hsp70 chaperone proteins play an essential role in assisting protein folding and preventing aggregation. They are also involved in transporting proteins into organelles such as mitochondria and the endoplasmic reticulum. This statement is accurate. E: Act only on fully synthesized polypeptide chains. Chaperone proteins can act on various stages of protein synthesis, including partially synthesized polypeptide chains. Therefore, this statement is inaccurate.

Identifying the Correct Statement

Based on the evaluation of each statement, we conclude that the accurate statement is: D: Of the Hsp70 class are involved in transport of proteins across mitochondrial and endoplasmic reticulum membranes.

Key concepts

Protein Folding

Proteins are essential building blocks in all living organisms, playing critical roles in virtually every biological process. The function of a protein is directly determined by its three-dimensional shape, often referred to as its conformation. Protein folding is the process by which a polypeptide chain acquires its native three-dimensional structure, a conformation that is typically biologically functional.

During the course of protein synthesis, folding occurs as a highly regulated process that leads to the unique, stable structure of each protein. This is crucial because misfolded proteins can form aggregates that may lead to diseases such as Alzheimer's and Parkinson's. Although proteins can fold unassisted because the correct fold represents a minimum energy state, in the crowded cellular environment, they often require assistance to avoid misfolding and to fold quickly enough to perform their functions in a timely manner.

The folding of proteins is guided by several types of bonds and interactions, including hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals forces. These interactions are influenced by the amino acid sequence of the protein and its chemical and physical environment.

Hsp70 Chaperone Proteins

Among the various chaperone proteins, Hsp70 (Heat Shock Protein 70) family plays an integral role. These proteins assist nascent polypeptides to fold, help protect cells from stress, and facilitate the refolding of misfolded proteins. They are named 'Hsp70' because they were first identified in response to heat shock and are approximately 70 kilodaltons in size.

The Hsp70 proteins have a unique way of binding to and stabilizing their substrate proteins through an ATP-driven process. They have a peptide-binding domain that holds the unfolded polypeptide, and an ATPase domain, which hydrolyzes ATP to provide the energy for the unfolding and refolding cycles necessary for proper protein conformation.

Role in Preventing Aggregation

During their functioning, they transiently bind to the nascent or misfolded proteins, preventing premature folding, aggregation, or targeting proteins for degradation if they're irreversibly damaged.

ATP in Protein Folding

ATP, or adenosine triphosphate, acts as the primary energy currency of the cell, and it also plays a pivotal role in the process of protein folding. Chaperones like Hsp70 require ATP to regulate their activity. The binding and hydrolysis of ATP to ADP within the chaperone proteins result in conformational changes that are integral to their function.

When ATP is bound, Hsp70 chaperone proteins typically have a low affinity for polypeptide chains, allowing them to bind loosely to the protein substrate. Once ATP is hydrolyzed to ADP, the affinity increases, and the chaperone binds more tightly to the unfolded protein. This tight binding is necessary for protecting the polypeptide chain from aggregation and for the correct folding to occur. Eventually, when ADP is released and replaced by new ATP, the protein substrate is released in a more folded state, ready for subsequent folding stages or for functioning if folding is complete.

Protein Transport

Proteins are often synthesized in one location within the cell, but many need to be transported to a specific organelle to perform their biological functions. Protein transport is the process by which proteins are directed to and translocated into different cellular compartments. For example, some proteins synthesized in the cytosol need to be transported into the mitochondria or the endoplasmic reticulum.

Hsp70 chaperone proteins assist in this transport process. They bind to the unfolded or partially folded polypeptide chains and prevent them from misfolding or aggregating. Hsp70 proteins also help in threading the polypeptide chains through narrow channels, such as those found in mitochondrial and endoplasmic reticulum membranes. Once inside the correct compartment, the proteins can complete their folding with the help of other chaperones that reside within that particular organelle.