Proteins: What They Are, Functions, Types and Structure
Updated: 3 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026
Quick Answer: What are proteins?
Proteins are molecules made of amino acids that fold into specific three-dimensional shapes. They are inside cells throughout the body and act as its cellular workhorses: providing structure, moving substances, catalyzing reactions, carrying messages, transporting molecules and helping defend against germs. [1]
Core idea | Plain-language meaning |
Building blocks | Amino acids joined by chemical bonds. |
Location | Cells throughout the body. |
Shape | One or more chains fold into a specific three-dimensional form. |
Main roles | Structure, transport, signaling, defense, movement and chemical reactions. |
Dietary meaning | Protein is a macronutrient found in many foods and drinks. |
Continuous process | Cells constantly make new proteins through protein synthesis. |
What the research says
Extra protein adds a small gain in muscle and strength when combined with resistance training. A systematic review and meta-analysis of 49 randomized trials with 1,863 healthy adults found that protein supplementation during resistance training increased fat-free mass by 0.30 kg (95% CI 0.09–0.52) and one-repetition-maximum strength by 2.49 kg (95% CI 0.64–4.33). Total protein intakes above about 1.62 g/kg/day produced no further gains in fat-free mass (Morton et al., Br J Sports Med, 2018). [2]
Higher protein intake was linked to lower death risk in large cohort studies, especially plant protein. A systematic review and dose-response meta-analysis of prospective cohort studies (31 studies in the meta-analysis, 715,128 participants, 113,039 deaths) found a pooled effect size for all-cause mortality of 0.94 (95% CI 0.89–0.99) for the highest versus lowest total protein intake. For plant protein it was 0.92 (95% CI 0.87–0.97), and an additional 3% of energy from plant protein per day was associated with a 5% lower risk of death from all causes. These are observational associations, not proof of cause and effect (Naghshi et al., BMJ, 2020). [3]
What are proteins?
Proteins are molecules inside every cell that help the body function and stay alive. They are made of amino acids and fold into complex three-dimensional shapes suited to specific tasks. [1]
A molecule is a particle made of one or more elements. A cell is the smallest unit of life. Proteins help cells create new cells and molecules, including other proteins.
Protein versus dietary protein
In nutrition, "protein" usually refers to the macronutrient found in foods and drinks. In biology, "proteins" refers to the individual molecules that work inside cells and tissues.
When a person eats beans, salmon, eggs or other protein-containing foods, the digestive system breaks dietary protein into amino acids. The body then uses those amino acids to build new proteins as part of metabolism. [1]
How much protein a person needs depends on factors that include age, activity level, body weight and daily calorie needs. This article does not calculate an individual protein target. [1]
What do proteins do in the body?
Proteins perform many different tasks, and no single protein performs every role. They are often described as the body's workhorses.
Function | Example |
Gene-related work | Help cells read genes and use the information to build cells, molecules and proteins. |
Structure | Provide support to cells and tissues. |
Defense | Attach to bacteria and viruses to help clear invaders. |
Digestion | Enzymes break down fats, proteins and carbohydrates into usable forms. |
Signaling | Send chemical signals and information from cell to cell. |
Chemical reactions | Catalyze reactions that help create energy for cells. |
Cell transport | Form channels that allow substances to enter or leave cells. |
Body transport | Hemoglobin carries oxygen to tissues through the blood. |
What are the main types of proteins?
Protein categories are based on what proteins do. Some proteins fit more than one broad category or perform additional roles.
Defense proteins
Defense proteins help protect the body. Antibodies are one example; they help the body ward off germs and other invaders. [1]
Enzymes
Enzymes catalyze chemical reactions such as digestion and can make reactions proceed at least 1 million times faster. Examples include lipase, carbohydrase, trypsin, thrombin and pepsin. [1]
Messenger proteins
Messenger proteins send chemical signals between cells, tissues and organs. Certain hormones are classified as messenger proteins because of the work they do in body processes. [1]
Motor proteins
Motor proteins help things move within and between cells. They support movement involved in wound healing, muscle movement, food movement through the digestive tract and blood flow. [1]
Structural proteins
Structural proteins support cells and help cells and substances move within the body. Collagen, tubulin and actin are examples. Tubulins join to form tubes that transport substances inside cells, and actin works with motor proteins to help muscles contract. [1]
Transport and channel proteins
Transport and channel proteins act like doorways that allow molecules to move across a cell membrane or between parts of a cell. They are often embedded in the cell membrane. [1]
What are proteins made of?
Proteins are made of amino acids held together by chemical bonds. One protein typically contains hundreds of individual amino acids. A single chain of amino acids is called a polypeptide, and one or more polypeptide chains twist and fold into a protein's unique three-dimensional shape. [1]
Amino acids can be compared with beads on a necklace: the order of the beads and the way the necklace folds affect the final structure.
What are the four levels of protein structure?
Protein structure ranges from the amino-acid sequence to the interaction of multiple folded chains.
Level | What it describes |
Primary | The amino-acid sequence in one polypeptide chain. |
Secondary | Local shapes created by chemical interactions, such as an alpha helix or beta sheet. |
Tertiary | The complete three-dimensional shape formed when secondary structures fold together. |
Quaternary | Multiple polypeptide chains interacting and folding together; not all proteins have this level. Hemoglobin is an example. |
A protein's exact structure helps determine its function. Changes to the expected structure can affect how a protein works and can contribute to health problems.
What is protein synthesis?
Protein synthesis is the process of making new proteins. It happens continuously inside cells because proteins break down over time and must be replaced to meet the body's needs. [1]
Cells use dietary amino acids as building blocks. DNA provides instructions, RNA helps carry and interpret those instructions, and ribosomes assemble amino acids into protein chains. [1]
Transcription
Transcription is the step in which DNA instructions are copied onto messenger RNA. It happens in the cell nucleus. [1]
Translation
Translation is the step in which ribosomal RNA interprets the instructions and directs the assembly of amino acids into polypeptide chains. Those chains then twist and fold into the shape required for the protein's job. [1]
Component | Role in protein synthesis |
DNA | Stores instructions for making proteins. |
Messenger RNA | Carries a copy of instructions from the nucleus to an assembly site. |
Transfer RNA | Delivers amino acids to ribosomes. |
Ribosomal RNA | Reads instructions and directs assembly. |
Ribosomes | Assemble amino acids into chains. |
Folded protein | The resulting molecule takes a shape suited to its function. |
What happens when protein structure changes?
The body relies on proteins adopting expected structures so they can do their jobs. Even a small change in a protein's structure can have significant effects.
Protein-related problems may happen when a gene change alters the body's instructions or when a protein folds incorrectly. Misfolded proteins may clump together or fail to work as expected.
What disorders are related to proteins?
Many protein-related conditions fall into two groups: genetic disorders and protein-folding disorders. There can be overlap between the categories.
Genetic disorders
A genetic disorder occurs when a gene change, or mutation, alters the body's instructions. Cells may then make proteins that do not look or act as they should, and effects can range from mild to life-threatening. Sickle cell disease is one example linked to a change in protein structure.
Protein-folding disorders
A protein-folding disorder occurs when a protein does not fold into its expected shape. Examples include Alzheimer's disease, Parkinson's disease and type 2 diabetes. [1]
Cystic fibrosis is described as both a genetic disorder and a protein-folding disorder. Protein-folding disorders often develop with aging, and biological changes associated with aging and epigenetic factors may contribute. [1]
Where are proteins located?
Proteins are in cells throughout the body. A given cell contains several thousand proteins, with each one carrying out a specific job that supports the body's needs. This helps explain why protein biology is connected to many systems: proteins can be involved in movement, structure, signaling, defense, digestion, transport and energy-related reactions. [1]
How does dietary protein support protein synthesis?
Dietary protein is broken down into amino acids by the digestive system. Cells use those amino acids as building blocks for new proteins. Protein needs vary by age, activity level, body weight and daily calorie needs, and a healthcare professional or dietitian can help create an eating plan when dietary protein intake is a concern. This overview does not recommend a specific amount of protein, supplement or eating pattern. [1]
When should you talk with a healthcare professional?
Talk with a healthcare professional if you are concerned about dietary protein intake, symptoms related to a known genetic disorder, or a suspected protein-folding disorder. A provider may refer you to a dietitian for an eating plan or explain what to expect with a protein-related disorder. Do not use an online article to diagnose a genetic or protein-folding disorder or to set an individualized protein target.
Conclusion: Proteins are specialized cellular workhorses
Proteins are amino-acid molecules that fold into shapes suited to specific tasks. They are found throughout the body and inside every cell. They provide structure, transport molecules, send signals, catalyze reactions, support movement and help defend against invaders. Dietary protein supplies amino acids that cells use to build new proteins. [1]
Protein synthesis is continuous and depends on DNA instructions, RNA molecules, ribosomes, amino acids and proper folding. Genetic changes and misfolding can affect protein function and contribute to health problems. Use this guide to prepare questions for a qualified healthcare professional or dietitian. Clear biology and nutrition information is a starting point for better care, not a diagnosis or personalized eating plan. [1]
Frequently Asked Questions
What are proteins?
Proteins are molecules made of amino acids. They fold into three-dimensional shapes and perform specialized jobs inside cells and throughout the body.
What are proteins made of?
Proteins are made of amino acids held together by chemical bonds. One protein typically contains hundreds of amino acids.
What is dietary protein?
Dietary protein is the macronutrient found in many foods and drinks. The digestive system breaks it into amino acids that the body uses to build new proteins.
What do proteins do?
Proteins provide structure, help defend against invaders, catalyze reactions, send signals, transport molecules, form cell channels and support movement.
What is an amino acid?
An amino acid is a building block of proteins. Amino acids join into polypeptide chains that fold into proteins.
What is a polypeptide?
A polypeptide is a single chain of amino acids. One or more polypeptide chains can fold together to form a protein.
What are the four levels of protein structure?
The four levels of protein structure are primary, secondary, tertiary and quaternary structure. They describe increasingly complex aspects of sequence, local shape, overall folding and interaction between multiple chains.
What is protein folding?
Protein folding is the process by which amino-acid chains twist and fold into the three-dimensional shape that supports a protein's function.
What is protein synthesis?
Protein synthesis is the process of making new proteins inside cells. It includes transcription and translation.
What happens during transcription?
DNA instructions are copied onto messenger RNA in the cell nucleus.
What happens during translation?
Ribosomal RNA interprets instructions and directs amino acids to be assembled into polypeptide chains, which then fold into proteins.
What are enzymes?
Enzymes are proteins that catalyze chemical reactions, including reactions involved in digestion.
What are antibodies?
Antibodies are defense proteins that help the body ward off germs and other invaders.
What are structural proteins?
Structural proteins support cells and tissues. Examples include collagen, tubulin and actin.
What are transport or channel proteins?
Transport or channel proteins act as doorways that allow molecules to move across cell membranes or between parts of a cell.
Can protein structure affect health?
Yes. Genetic changes or protein misfolding can produce proteins that do not look or work as expected and may contribute to health problems.
What disorders are associated with protein folding?
Alzheimer's disease, Parkinson's disease and type 2 diabetes are examples. Cystic fibrosis is both a genetic disorder and a protein-folding disorder.
How much protein do I need?
Needs depend on age, activity level, body weight and daily calorie needs. A healthcare professional or dietitian can help with an individualized eating plan.
When should I talk with a healthcare professional?
Talk with one if you are concerned about dietary protein intake or a known or suspected genetic or protein-folding disorder.
Related Reading
Understanding Genes: The Blueprints of Life explains how DNA carries the instructions for making proteins.
Genetic Mutations: Understanding the Blueprint of Human Variation covers gene changes that can alter how proteins are built.
Cystic Fibrosis: Symptoms, Causes, Diagnosis, and Treatment (2026 Guide) explains a genetic disease caused by a faulty protein.
Alzheimer's Disease: Symptoms, Causes, Treatment & Complete Guide covers a brain disease linked to misfolded protein buildup.
References
Cleveland Clinic. What Are Proteins? Definition, Types & Examples. Cleveland Clinic. Updated June 24, 2025. https://my.clevelandclinic.org/health/body/proteins
Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376-384. PMID 28698222. https://doi.org/10.1136/bjsports-2017-097608 Erratum in: Br J Sports Med. 2020;54(19):e7. PMID 32943392. https://doi.org/10.1136/bjsports-2017-097608corr1
Naghshi S, Sadeghi O, Willett WC, et al. Dietary intake of total, animal, and plant proteins and risk of all cause, cardiovascular, and cancer mortality: systematic review and dose-response meta-analysis of prospective cohort studies. BMJ. 2020;370:m2412. PMID 32699048. https://doi.org/10.1136/bmj.m2412
Medical disclaimer
This article is for general educational purposes and is not a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified healthcare professional with any questions about a medical condition or treatment.
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