Protein Structures that are Changed By Heating a Protein

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Proteins are large molecules consisting of one or more chains of up to 20 different types of amino acids; these chains fold into a three-dimensional structure. Proteins are necessary for the body’s structure, regulation and functioning, serving as structural elements, enzymes, hormones and antibodies. The shape into which a protein naturally folds is called its “native state.” Such native shape determines its function.

A protein’s structure is comprised of three parts: the primary structure, the secondary structure and the tertiary structure. The primary structure is the amino acid sequence, which is held together by strong, covalent, peptide bonds. The secondary structure is the geometric shape that results from hydrogen bonding of the amide groups. Such structure may be in the form of an alpha helix, in which the protein chain is coiled tightly, resembling a corkscrew, and held together by intramolecular hydrogen bonding of the amino acids along the chain. Or the structure may be in the form of a beta-pleated sheet, in which individual chains are aligned side by side and held together by intermolecular hydrogen bonding of the amide groups of the two chains. The tertiary structure is the final overall shape of a single protein molecule, i.e., the relationship of the secondary structures to one another, as determined by a variety of bonding interactions. Such bonds are formed primarily by non-polar hydrophobic (water repelling) interactions, and also by hydrogen bonds, ionic interactions and disulfide bonds. In non-polar hydrophobic interactions, the tertiary structure is achieved by the chain of amino acids looping and folding over itself, so that the portion of the molecules with an affinity of water (the hydrophilic portions) are on the outside of the molecule, and the water repelling hydrophobic portions are buried in the internal parts of the molecule.

The term “denaturation” refers to the response of a protein to heat, acid, alkali and a variety of other physical and chemical agents. It is commonly defined as any noncovalent change in the structure of a protein that disrupts the secondary or tertiary structure of the molecules, causing the protein to uncoil or unravel into a random shape. Since denaturation is not strong enough to break the covalent peptide bonds, however, the primary structure (the sequence of amino acids) is not disrupted. When a protein is denatured, its biological activity and function are affected.

Heat denaturation occurs when protein molecules are heated to increased temperatures. By heating a protein to a certain temperature (which is protein specific), the molecules within the protein vibrate to an extent that is so rapid and violent that the bonds resulting in both the secondary and tertiary structures are weakened. The first bonds affected are those resulting in the tertiary structure, causing the protein to obtain a more flexible structure. If heating stops at this state, the protein should be able to readily refold to its native structure. As heating continues, however, the bonds resulting in the secondary structure weaken and are disrupted. As a result, the strings of amino acids forming the alpha helix or beta-pleated sheet unravel, disrupting the protein’s native shape and function.

With respect to heat denaturation, the temperatures at which various proteins uncoil vary greatly. While some proteins, such as the gluten proteins, unfold at very high temperatures, other proteins unfold at low temperatures that are only a few degrees higher than those at which they function.

There are many beneficial applications of heat denaturation. For example, by cooking, the proteins in food denature, thereby becoming easier to digest. In addition, heating medical supplies and instruments causes the proteins in bacteria to denature, thus destroying such bacteria. Denaturation, however, can also have negative effects. For example, in dairy processing, heat treatments cause certain proteins to be denatured, thus affecting the milk’s nutritional content.

List of Proteins

Fibrous proteins

Cytoskeletal proteins

Actin
Arp2/3
Coronin
Dystrophin
FtsZ %
Keratin
Myosin
Spectrin
Tau (protein)
Tubulin

Extracellular matrix proteins

Collagen
Elastin
F-spondin
Pikachurin

Globular proteins

Plasma proteins

Serum Amyloid P Component
Serum albumin

Coagulation factors

Complement proteins
- C1-inhibitor
- C3-convertase
Factor VIII
Factor XIII
Fibrin
Protein C
Protein S
Protein Z
Protein Z-related protease inhibitor
Thrombin
Von Willebrand Factor

Acute phase proteins

C-reactive protein

Hemoproteins

Cell adhesion

Cadherin
Ependymin
Integrin
NCAM
Selectin

Transmembrane transport proteins

CFTR
Glycophorin D
Scramblase

Ion channels

Acetylcholine receptor
- Muscarinic acetylcholine receptor — these are NOT ion channels, but rather G-protein coupled receptors; see below
- Nicotinic acetylcholine receptor
Potassium channel

Synport / Antiport proteins

Glucose transporter

Hormones and growth factors

Epidermal growth factor
Insulin
Insulin-like growth factor
Oxytocin

Receptors

Receptors with enzyme activity are in the enzymes section.
Receptors that are ion channels are in the ion channel section.

Transmembrane receptors

G-protein-coupled receptor
- Rhodopsin

Intracellular receptors

Estrogen receptor

DNA-binding protein

Histones

Transcription Regulation

CI protein %

Transcription regulatory proteins that are receptors are in the receptors section.

C-myc
FOXP2
FOXP3
MyoD
P53

Immune system proteins

Immunoglobins
Major histocompatibility antigens
T cell receptor

Nutrient storage/transport

Ferritin

Chaperone proteins

GroEL %

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