Protein Folding and Enzyme Assembly

Before an enzyme can catalyse anything, its newly synthesised polypeptide chain must fold into a precise three-dimensional shape, and many enzymes must further assemble several chains into a functional complex. Folding is driven by the amino-acid sequence but is assisted in the cell by molecular chaperones, and its failure can produce inactive or aggregation-prone proteins.

Definition

Protein folding is the process by which a polypeptide chain acquires its functional three-dimensional structure; enzyme assembly is the subsequent association of folded subunits (and any cofactors) into the complete, catalytically active enzyme.

Scope

The entry covers the thermodynamic basis of folding, chaperone-assisted folding and quality control, the assembly of subunits into quaternary structures, and the consequences of misfolding and aggregation. It treats folding and assembly as reference biochemistry and is not a source of clinical guidance.

Core questions

• What determines a protein's folded structure?
• How do molecular chaperones assist folding in the cell?
• How do enzymes assemble from multiple subunits?
• What happens when folding fails?

Key concepts

• Native state and free-energy minimum
• Sequence determines structure
• Molecular chaperones
• Quaternary assembly of subunits
• Protein misfolding and aggregation
• Amyloid formation
• Computational structure prediction

Key theories

Anfinsen's thermodynamic hypothesis

Under physiological conditions a protein's native, lowest-free-energy conformation is encoded by its amino-acid sequence, so the folding information needed for function resides in the sequence itself.

Mechanisms

A nascent polypeptide samples conformations and collapses toward its native, lowest-free-energy state, with the sequence specifying that state under cellular conditions. Because crowded cells favour misfolding and aggregation, molecular chaperones bind exposed hydrophobic regions, prevent inappropriate associations, and give chains repeated chances to fold correctly, while quality-control systems degrade those that fail. Many enzymes then assemble their folded subunits, and bind required cofactors, to form the active holoenzyme. When folding goes wrong, proteins can lose activity or convert into ordered aggregates such as amyloid fibrils. Advances in computational prediction now allow accurate inference of folded structures directly from sequence.

Clinical relevance

Protein misfolding and aggregation are features of a range of human diseases, so understanding folding and its quality control is important background for biomedical research. This entry describes the biology of folding and assembly for reference and is not a basis for diagnosis or treatment.

History

Anfinsen's refolding experiments, synthesised in his 1973 account, established that sequence encodes structure and posed the puzzle of how folding occurs so rapidly. The subsequent discovery of molecular chaperones showed that, although thermodynamics governs the destination, the cell actively assists the path and guards against aggregation (Tyedmers and colleagues, 2010). Work on misfolding linked aberrant folding to amyloid disease (Chiti and Dobson, 2006; Knowles and colleagues, 2014), and deep-learning methods later achieved accurate structure prediction from sequence (Jumper and colleagues, 2021).

Key figures

• Christian B. Anfinsen
• Christopher M. Dobson
• F. Ulrich Hartl

Seminal works

• anfinsen-1973
• chiti-2006
• jumper-2021

Frequently asked questions

Does a protein's sequence really contain all the information needed to fold?

Under physiological conditions the native structure is the sequence's lowest-free-energy state, so the folding information is encoded in the sequence; in the crowded cell, however, chaperones are often needed to reach that state efficiently and avoid aggregation.

Why does enzyme assembly matter for activity?

Many enzymes are active only as folded, multi-subunit complexes with their cofactors in place; correct folding and assembly are prerequisites for forming the intact active site.