[proto- + -in]
Any of a class of complex nitrogen-containing compounds synthesized by all living organisms and yielding amino acids when hydrolyzed. Dietary proteins provide the amino acids necessary for the growth and repair of animal tissue.

All amino acids contain carbon, hydrogen, oxygen, and nitrogen; some also contain sulfur. About 20 different amino acids make up human proteins, which may contain other minerals such as iron or copper. A protein consists of from 50 to thousands of amino acids arranged in a specific sequence. The essential amino acids are those the liver cannot synthesize (tryptophan, lysine, methionine, valine, leucine, isoleucine, phenylalanine, threonine, arginine, and histidine); because they cannot be made by the body, they must be consumed as food. A protein containing all of them is called a complete protein. An incomplete protein lacks one or more of the essential amino acids. The nonessential amino acids are synthesized by the liver.

Milk, eggs, cheese, meat, fish, and some vegetables such as soybeans are the best dietary sources of protein. Proteins are found in both vegetable and animal food sources. Many incomplete proteins are found in vegetables; they contain some but not all of the essential amino acids. A vegetarian diet can compensate for dietary protein deficiencies by combining vegetable groups that complement each other in their basic amino acid groups.

Principal animal proteins are lactalbumin and lactoglobulin in milk; ovalbumin and ovoglobulin in eggs; serum albumin in serum; myosin and actin in striated muscle tissue; fibrinogen in blood; serum globulin in serum; thyroglobulin in thyroid; globin in blood; thymus histones in thymus; collagen and gelatin in connective tissue; collagen and elastin in connective tissue; and keratin in the epidermis. Chondroprotein is found in tendons and cartilage; mucin and mucoids are found in various secreting glands and animal mucilaginous substances; caseinogen in milk; vitellin in egg yolk; hemoglobin in red blood cells; and lecithoprotein in the blood, brain, and bile.

Ingested proteins are a source of amino acids needed to synthesize the body's own proteins, which are essential for the growth of new tissue or the repair of damaged tissue; proteins are part of all cell membranes. Excess amino acids in the diet may be changed to simple carbohydrates and oxidized to produce adenosine triphosphate and heat; 1 g supplies 4 kcal of heat.

Infants and children require from 2 to 2.2 g of dietary protein per kilogram of body weight each day for normal health and development. The World Health Organization recommends that healthy adults consume about 0.8 g of protein per kg of body weight daily. The calculation should be made on the basis of ideal body weight rather than the actual weight of the adult or child. High levels of exercise, menstruation, pregnancy, lactation, and convalescence from severe illness require increased protein intake. Excess protein in the diet results in increased nitrogen excretion in the urine.

accessory protein

A protein that works with another protein, e.g., in helping it to fold into its normal shape or become anchored into its preferred location in a cell membrane.

activator protein

A protein that stimulates the expression of a gene.

acute phase protein

Any of the plasma proteins whose concentration increases or decreases by at least 25% during inflammation. Acute-phase proteins include C-reactive protein, several complement and coagulation factors, transport proteins, amyloid, and antiprotease enzymes. They help mediate both positive and negative effects of acute and chronic inflammation, including chemotaxis, phagocytosis, protection against oxygen radicals, and tissue repair. In clinical medicine the erythrocyte sedimentation rate or serum C-reactive protein level sometimes is used as a marker of increased amounts of acute-phase proteins.
SYN: SEE: acute phase reactant
SEE: inflammation

adapter protein

An intracellular molecule that undergoes structural and functional changes in response to binding of cell membrane receptors by ligands. Adapter proteins participate in the immune response by acting as a bridge for enzymes in the signaling pathway needed to activate lymphocytes and initiate a response to an antigen.

amyloid precursor protein

ABBR: APP An integral membrane protein concentrated at neuron synapses that is cleaved biochemically into components, one of which is the Alzheimer disease-associated beta amyloid. Mutations in the gene for APP on chromosome 21 account for less than 5% of early-onset familial Alzheimer disease.

autotransporter protein

Any of a large family of proteins that are found in gram-negative bacteria and facilitate the secretion of toxic substances and virulence factors from inside the bacterial cell to the outer cell membrane.

Bence Jones protein

SEE: Bence Jones protein

binding protein

A protein that is linked to another chemical in the body, either transporting it through the blood or helping to convey it into cells across cell membranes. Examples of binding proteins include ceruloplasmin, sex hormone-binding globulin, and transcobalamin, which carries vitamin B12.

blood protein

A broad term encompassing numerous proteins, including hemoglobin, albumin, globulins, the acute-phase reactants, transporter molecules. Normal values are hemoglobin, 13 to 18 g/dL in men and 12 to 16 g/dL in women; albumin, 3.5 to 5.0 g/dL of serum; globulin, 2.3 to 3.5 g/dL of serum. The amount of albumin in relation to the amount of globulin is referred to as the albumin-globulin (A/G) ratio, which is normally 1.5:1 to 2.5:1.

bone morphogenetic protein

A bone graft substitute.

breast cancer resistance protein

Any protein that stimulates cells to excrete chemotherapeutic drugs and/or limits their absorption from the intestines.

protein C

A plasma protein that inhibits coagulation factors V and XIII, preventing excessive clotting. Deficiency of this protein or resistance to its effects may lead to deep venous thrombosis and pulmonary embolism. In the presence of thrombin, protein C is activated, forming activated protein C.
SEE: protein S

carrier protein

1. A protein that elicits an immune response when coupled with a hapten.
2. A membrane protein that helps a specific substance diffuse into a cell.

CHK2 protein

CHEK2 protein An abbreviation for a checkpoint kinase protein that stimulates cells to multiply and is found in excessive amounts in several cancers, including cancers of blood, breast, stomach, and vulva.

cholesteryl ester transfer protein

ABBR: CETP A protein that circulates in plasma and facilitates the chemical transfer of cholesteryl esters from high-density lipoproteins to other lipoproteins.

coagulated protein

One of the derived (insoluble) proteins resulting from the action of alcohol, heat, or other physicochemical entities on protein solutions.

coat protein

SEE: Capsid.

complement S protein

SEE: Vitronectin.

complete protein

A protein containing all the essential amino acids.

conjugated protein

A protein that is chemically linked with a nonprotein molecule. Included are chromoproteins (such as hemoglobin); glycoproteins (such as mucin); lecithoproteins, nucleoproteins, and phosphoproteins (such as casein).

C-reactive protein

ABBR: CRP The first acute-phase protein identified. It binds with phospholipids on foreign substances, activates the complement system, stimulates the production of cytokines, and inhibits the production of oxygen radicals by neutrophils. Increased blood levels of CRP are present in many infectious and inflammatory diseases (including in patients with coronary artery disease, in whom it is sometimes employed as a risk factor). CRP levels are sometimes monitored serially to determine if infectious or inflammatory diseases have been effectively treated.
SEE: acute phase protein

decorin-binding protein

One of two antigens released by Borrelia burgdorferi (the spirochete that causes Lyme disease). The antigen may be useful as a target for Lyme disease vaccination.

denatured protein

A protein in which the amino acid composition and stereochemical structure (shape) have been altered by physical or chemical means.
SEE: coagulated protein

derived protein

A protein altered chemically or physically.

fusion protein

A protein made from the natural or artificial hybridization and translation of two distinct genes.

G protein

Any of a family of cellular proteins activated by the binding of an intercellular signal (guanine nucleotides) to its receptor on the cell membrane. The G-protein then activates the enzyme adenyl cyclase within the cell, triggering the formation of cyclic adenosine monophosphate and stereotyped responses. Hormones that bind and activate cellular G proteins include adrenaline, angiotensin, glucagon, somatostatin, thyroid-stimulating hormone, and vasopressin.

glial fibrillary acidic protein

ABBR: GFAP An intermediate filament found only in astrocytes and astroglial cells. It forms part of the skeletal structure of neurons. Abnormalities in GFAP are found in a variety of neurological diseases, including Alzheimer disease, Creutzfeldt-Jakob disease, and some brain tumors.

glycosyl phosphatidyl inositol anchor protein

Any of the structural components of the red blood cell membrane. Their absence from cells is responsible for cellular destruction (hemolysis) in paroxysmal nocturnal hemoglobinuria.

heart fatty acid-binding protein

heart-type fatty acid-binding protein ABBR: H-FABP A 15-kDa protein found in cardiac cells. Its small size, rapid release from damaged tissues, and abundance in the heart makes its detection in the blood a useful biomarker in assessing patients with chest pain as indicative of an acute coronary syndrome.

heat shock protein

Any of a large group of proteins that protect cells from injury caused by increased temperatures or other physical stresses.
SYN: SEE: stress response protein

heparin-binding protein

A proinflammatory, antimicrobial peptide released by neutrophils. It increases the permeability of blood vessels. It is found in high concentrations in plasma of patients with septic shock.

HFE protein

Human hemochromatosis protein, normally found intracellularly in duodenal crypt enterocytes and the placenta. It is closely associated with transferrin receptors for iron and regulates iron absorption. HFE is a homologue of class I major histocompatibility complex molecules. The C2824 and H63D mutations on chromosome 6 in the HFE gene cause hemochromatosis.

high-mobility-group box chromosomal protein 1

ABBR: HMGB1 A component of chromatin, released as a cytokine in the systemic inflammatory response of sepsis.

human hemochromatosis protein

SEE: HFE protein

immune protein

An antibody or immunoglobulin produced by plasma cells that identifies foreign antigens and initiates their destruction.

incomplete protein

A protein lacking one or more of the essential amino acids.
SEE: amino acid, essential

intact protein

Non-hydrolyzed protein, i.e., protein that has not been broken down into peptides or individual amino acids.

lipopolysaccharide-binding protein

One of many acute-phase proteins released into the serum in patients with a gram-negative bacterial infection. The protein helps defend the body against sepsis by binding and transferring bacterial endotoxin.

M protein

A protein in the cell wall of group A streptococci that helps inhibit the ingestion of bacteria by polymorphonuclear white blood cells. Some of the more than 80 identified M proteins have been linked to poststreptococcal acute rheumatic fever. Others have been linked to poststreptococcal acute glomerulonephritis.

membrane-bound protein

membrane protein A protein that is part of a cell membrane and acts as a receptor for substances transported in extracellular fluid or as an agent that mediates the transport of chemicals into or out of the cell.

motor protein

An intracellular protein (such as actin, dynein, or myosin) that changes the length or flexibility of intracellular structures. Motor proteins contribute to the beating of cilia and flagella, the contraction of muscles, the movement of vesicles, the streaming of cytoplasm, and the formation of spindles during meiosis and mitosis.

native protein

A protein in its natural state and has not been denatured.

nuclear matrix protein 22

ABBR: NMP22 A tumor marker excreted in the urine of some patients with bladder cancer. NMP22 can be used to screen patients for the disease, esp. for recurrence of the disease after primary treatment.

nucleotide-binding oligomerization domain protein

ABBR: NOD Any of a family of cytoplasmic proteins that recognize molecules associated with disease-causing bacteria and stimulate cells to secrete cytokines and costimulatory molecules. Inappropriate regulation of NODs has been linked to the pathogenesis of Crohn disease and other syndromes.

oncofetal protein

SEE: Oncofetal antigen.

plasma protein

A protein present in blood plasma, such as albumin or globulin.

pregnancy-associated plasma protein A

ABBR: PAPP-A A plasma protein used as a screening test between 8 and 14 weeks' gestation. Diminished levels of the protein suggest an increased risk for Down syndrome, intrauterine growth retardation, preeclampsia, and stillbirth.

protein S

A vitamin-K-dependent protein that acts with protein C to prevent blood clotting. Deficiencies may lead to venous thrombosis and pulmonary embolism.
SEE: protein C

sarcomeric protein

Any of the proteins in a sarcomere that contribute to the contraction of a muscle fiber. These proteins include actin, myosin, troponin, and nebulin.

serum protein

Any protein in the blood serum. The two main fractions are albumin and the globulins. Serum protein forms weak acids mixed with alkali salts. This increases the buffer effects of the blood but to a lesser extent than does cellular protein.

simple protein

Any of the proteins that produce alpha amino acids on hydrolysis, e.g., albumins, albuminoids, globulins, glutelins, histones, prolamines, and protamines.

soy protein

A vegetable protein found in food products derived from soybeans. Soy-based foods also contain fiber, flavones, phytoestrogens, and other potentially beneficial components. Consumption of soy protein instead of animal protein may reduce the risk of gout.
SEE: soy milk; SEE: tofu

steroidogenic acute regulatory protein

ABBR: StAR protein A protein found within cells of the adrenal glands and gonads that stimulates the conversion of cholesterol into sex hormones, corticosteroids, and mineralocorticoids.

stress response protein

SEE: Heat shock protein.

transport protein

One of the proteins important in transporting materials such as hormones from their site of origin to the site of cellular action and metabolism.

vascular cell adhesion protein

SEE: Vascular cell adhesion molecule-1.

protein Z

A vitamin-K-dependent coagulation protein that helps thrombin bind to phospholipids, a critical step in the coagulation cascade. It is made in the liver and circulates in the blood.