Last updated: October 31, 2026 ยท Written by Dr Nidhi Joshi, MSc, PhD
In short: Proteins are large molecules made of amino acids linked in a chain that folds into a precise 3D shape. The main types of proteins are grouped by function (enzymes, structural, transport, hormones, antibodies, motor, storage and receptors), by shape (fibrous, globular, membrane) and by make-up (simple or conjugated).
What is a protein?
A protein is a chain of amino acids joined by peptide bonds. Cells use 20 standard amino acids, and the order in which they are joined is written in a gene. A short chain (up to a few dozen amino acids) is usually called a peptide; a longer one is a polypeptide. It becomes a working protein once it folds into its final shape.
That shape is what lets a protein do its job. An enzyme’s pocket fits only its substrate, and an antibody’s tips fit only one antigen. Because 20 building blocks can be arranged in almost endless orders, proteins can take on thousands of different shapes and jobs. The different types of proteins are simply different answers to the question: what does this shape let the protein do?
The 4 levels of protein structure
- Primary structure: the sequence of amino acids, read from the gene. Change one amino acid and the whole protein can behave differently.
- Secondary structure: short stretches fold into regular patterns held by hydrogen bonds, mainly the coiled alpha helix and the pleated beta sheet.
- Tertiary structure: the complete 3D shape of one chain, held together by interactions between side chains (hydrophobic packing, hydrogen bonds, ionic bonds and disulfide bridges).
- Quaternary structure: two or more chains fitted together. Hemoglobin, for example, is made of four chains (two alpha and two beta).
Types of proteins and their functions
The most useful way to sort proteins is by what they do. This table covers the main types of proteins with everyday examples.
| Type | What it does | Examples |
|---|---|---|
| Enzymes | Speed up chemical reactions | Amylase, pepsin, DNA polymerase |
| Structural | Give strength and shape to cells and tissues | Collagen, keratin, tubulin |
| Transport | Carry substances in the blood or across membranes | Hemoglobin, albumin, ion channels |
| Hormones and messengers | Carry signals between cells and organs | Insulin, glucagon, growth hormone |
| Defence (antibodies) | Recognise and help remove germs; seal wounds | Immunoglobulin G (IgG), fibrin |
| Motor (contractile) | Make muscles contract and move cargo in cells | Actin, myosin, kinesin |
| Storage | Store nutrients or metal ions | Ferritin (iron), casein (milk), ovalbumin (egg white) |
| Receptors | Detect signals and pass them into the cell | Insulin receptor, rhodopsin (light) |
Two of these groups have their own guides on NJBiome. Read what enzymes are made of and how they work, and see how protein hormones such as insulin compare with steroid hormones in hormones and their functions. Antibodies are covered in the immune system and its parts.
Fibrous vs globular proteins
Proteins can also be sorted by overall shape. Fibrous proteins form long strands or sheets, while globular proteins fold into compact balls.
| Fibrous proteins | Globular proteins | |
|---|---|---|
| Shape | Long, rope- or sheet-like | Compact, roughly round |
| Water solubility | Mostly insoluble | Usually soluble |
| Main role | Strength and support | Active jobs: catalysis, transport, signalling, defence |
| Sequence | Often repetitive | Irregular, varied |
| Examples | Collagen, keratin, elastin, silk fibroin | Enzymes, hemoglobin, insulin, antibodies |
A third group, membrane proteins, sits inside the fatty membrane around every cell. Channels, pumps and receptors belong here; they let ions and signals cross a barrier that water-soluble molecules cannot pass on their own.
Simple vs conjugated proteins
A simple protein contains only amino acids. Albumin in blood and many digestive enzymes are examples. A conjugated protein also carries a non-protein part, called a prosthetic group, that it needs to work:
- Hemoproteins: hemoglobin and myoglobin hold a heme group with iron that binds oxygen.
- Glycoproteins: carry sugar chains; many antibodies and cell-surface markers are glycoproteins.
- Lipoproteins: carry fats such as cholesterol in the blood (LDL and HDL).
- Metalloproteins: hold metal ions, such as zinc in carbonic anhydrase or iron in ferritin.
- Nucleoproteins: bind DNA or RNA, like the histones that package DNA in chromosomes.
When proteins misfold
Because function depends on shape, a protein that loses its shape stops working. Heat, strong acid or alcohol can unfold (denature) proteins; that is why egg white turns solid and white when cooked.
Small changes inside the body can cause disease. In sickle cell disease, a single amino acid change in hemoglobin makes the molecules stick together and bend red blood cells into a sickle shape. In Alzheimer’s disease, misfolded amyloid-beta and tau proteins build up in the brain. Prion diseases, such as Creutzfeldt-Jakob disease, are caused by a misfolded protein that pushes normal copies to misfold too.
Proteins in medicine and biotechnology
Many modern medicines are proteins. Human insulin and growth hormone are made by bacteria or yeast using recombinant DNA technology. Monoclonal antibodies treat some cancers and autoimmune diseases, and purified enzymes are used in diagnosis and to replace missing enzymes. Even mRNA vaccines work through proteins: they give cells instructions to make one harmless viral protein so the immune system can learn to recognise it.
Related: what a peptide drug is and how it differs from a protein
Key takeaways
- Proteins are chains of 20 kinds of amino acids that fold into a shape that decides their job.
- Structure has four levels: sequence, local folds, the full 3D shape and multi-chain assemblies.
- By function, the main types of proteins are enzymes, structural, transport, hormones, antibodies, motor, storage and receptor proteins.
- By shape they are fibrous, globular or membrane proteins; by make-up, simple or conjugated.
- Misfolded proteins can cause disease, and engineered proteins are now major medicines.
Frequently asked questions
What are the main types of proteins?
Grouped by function, the main types of proteins are enzymes, structural proteins, transport proteins, hormones and other messengers, antibodies, motor proteins, storage proteins and receptors. Proteins can also be grouped by shape (fibrous, globular, membrane) or by make-up (simple or conjugated).
What are the 4 levels of protein structure?
Primary structure is the order of amino acids. Secondary structure is local folding into alpha helices and beta sheets. Tertiary structure is the full 3D shape of one chain. Quaternary structure is how two or more chains fit together, as in hemoglobin.
What is the difference between fibrous and globular proteins?
Fibrous proteins are long, rope-like and mostly insoluble in water; they give strength and support, like collagen and keratin. Globular proteins are compact, round and usually water-soluble; they do active jobs, like enzymes, hemoglobin and antibodies.
What is a conjugated protein?
A conjugated protein is a protein joined to a non-protein part called a prosthetic group. Hemoglobin carries heme, glycoproteins carry sugars, lipoproteins carry fats and metalloproteins hold metal ions.
Are all enzymes and hormones proteins?
Almost all enzymes are proteins, although a few RNA molecules (ribozymes) also act as catalysts. Many hormones are proteins or peptides, such as insulin and growth hormone, but steroid hormones like cortisol and estrogen are made from cholesterol and are not proteins.
This article is for general education only and is not medical advice. Talk to a doctor about any health concern.
Sources
- OpenStax Biology 2e. 3.4 Proteins.
- MedlinePlus Genetics. What are proteins and what do they do?
- Cleveland Clinic. Proteins: What They Are and Why You Need Them.
- Alberts B, et al. Molecular Biology of the Cell, 4th ed. (NCBI Bookshelf). The Shape and Structure of Proteins.
- Nature Education Scitable. Protein Structure.