Protein: Brief description

Protein is one of the nutrients along with carbohydrate, fat, vitamins, minerals, and water.

Proteins are single, unbranched chains of amino acid monomers. Amino acids are the building blocks of proteins. There are 20 different naturally occurring amino acids. The amino acid side chains in a peptide can become modified, extending the functional repertoire of amino acids to more than hundred different amino acids. A protein’s amino acid sequence determines its three-dimensional structure.

To make a protein, these amino acids are joined together in a polypeptide chain through the formation of a peptide bond.

Chains that are less than 40-50 amino acids or residues are often referred to as polypeptide chains since they are too small to form a functional domain. Larger than this size, they are called proteins.

Humans can synthesize 12 (nutritionally nonessential) of the 20 common amino acids from the amphibolic intermediates of glycolysis and of the citric acid cycle. Of the 12 nutritionally nonessential amino acids, nine are formed from amphibolic intermediates and three (cysteine, tyrosine and hydroxylysine) from nutritionally essential amino acids. Protein is an essential nutrient. There is no life without protein.

Protein is contained in every part of human body, the skin, muscles, hair, blood, body organs, eyes, even fingernails and bone. Next to water, protein is the most plentiful substance in human body.

Protein has a critical physiological function. Protein is primarily used in the body to build, maintain, and repair body tissues. In the event that protein intake is greater than that required by the body for this primary function, excessive protein is converted to energy for immediate use or stored in the body as fat.
Protein: Brief description 

Amino acids – fundamental units of protein

Amino acids, peptides and proteins are important constituents of food. They supply the required building blocks for protein biosynthesis.

 The amino acids are the fundamentals units of protein structure. All amino acids contain at least one amino group (-NH2) in the alpha position and one carboxyl, and all (except Glycine) contain an asymmetric carbon atom. For this reason, they may exist as isomers.

The term amino acid might mean any molecule containing both an amino group and any type of acid group; however, the term is almost always used to refer to an carboxylic acid. The simplest acid is aminoacetic acid, called glycine.

Amino acids play major role in regulating multiple processes related to gene expression, including modulation of the function of the proteins that mediate messenger RNA (mRNA) translation.

Most naturally occurring amino acids are of the L-configurations, although D-amino acids are not uncommon in some microorganisms. The presence of a D-amino acid oxidase in mammalian tissue, however, suggests that the D-forms may play some yet unrecognized role in mammalian protein metabolism.

Based on their nutritional/physiological roles, amino acids can be differentiated as:
• Essential amino acids: Valine, leucine, isoleucine, phenylalanine, tryptophan, methionine, threonine, histidine (essential for infants), lysine and arginine (“semi-essential”).
• Nonessential amino acids: Glycine, alanine, proline, serine, cysteine, tyrosine, asparagine, glutamine, aspartic acid and glutamic acid.

Amino acids are utilized in formation of protein. If amino acids are deficient, then protein synthesis does not occur. As a result protein deficiency disease may occur. It is necessary to take balanced diet containing all essential amino acids.
Amino acids – fundamental units of protein

What are the functions of protein and amino acids?

The dietary requirements of certain of the amino acids are influenced by the intake of other nutrients. For example, phenylalanine is converted to tyrosine in the animal cell.  Results from clinical experiments involving the oral administration of tyrosine or phenylalanine to patients with tyrosinosis led Grace Medes to conclude in 1932 that phenylalanine also can be converted into tyrosine in humans.

The dietary requirement for phenylalanine therefore is a function of the total aromatic amino acid content of the diet.

Similarly, methionine may function metabolically as a precursor of other sulfur-containing amino acids so that both of the dietary methionine and cystine determine the requirement for methionine.

The methionine metabolic pathway supports numerous functions within the central nervous system beyond providing an essential amino acid precursor for protein synthesis.

These functions include neurotransmitter synthesis, methyl group donation, polyamine precursor, osmotic protection, antioxidant synthesis, and DNA salvage synthesis.
The relationship between tryptophan and nicotinic acid is another important example. Tryptophan may be metabolized to form nicotinic acid, and in so doing, contributes to the total amount of the vitamin available for cellular metabolism.

Many of the amino acids are precursors of other significant compounds required in metabolic processes. For example, tyroxine and therefore, phenylalanine give rose to the hormones tyroxine and epinephrine.

L-arginine is a precursor for biosynthesis of other amino acids present in protein (glutamate and proline) or not present in proteins (ornithine, citruline). Arginine is a precursor of urea, polyamines, creatine and nitric acid. Polyamines are involved in cell mitosis.
 
Glutamic acid cysteine, and glycine are components of a tripeptide glutathione, which functions in cellular oxidation-reduction reactions. Glutamic acid can undergo conversion to alpha-ketoglutarate and ammonium. As such, glutamate is an oxidative substrate in several cell types.

Sulfur containing amino acids give rise to taurine a bile acid component. Tryptophan may be metabolized to form serotonin (5-hydroxytryptamine), a tissue hormone that is found predominantly in serum, blood platelets, gastrointestinal mucosa and nerve tissue.

Methionine provides methyl groups for synthesis of choline, creatine and methylation of nicotinamide to its major excretion product N’-methylnicotinamide.

Glycine contributes to the porphyrin ring of hemoglobin and, along with serine, provides part of the structure of the purine and pyrimidines of the nuclei acids.

Two hydroxylated amino acids – hydroxyproline and hydroxylysine – are important constituents of collagen; approximately 12 percent of the total amino acids content of collagen is hydroxyproline.
What are the functions of protein and amino acids?

Proteins and Amino Acid

The word protein comes from the Greek ‘proteios’ which means ‘of the first rank or importance’.

Nearly half of the dry weight of a typical animal cell is protein. Structural components of the cell, antibodies, and many of the hormones are proteins but as much as 90% of cellular proteins are the enzymes upon which fundamental cellular function depends. They may be as many as 1000 different enzymes in a single cell.

Protein are essential components of muscle, skin, cell membranes, blood, hormones, antibodies, enzymes and genetic material and almost all other body tissues and components.

The protein molecule is a polymer of amino acids joined in peptide linkages. Nitrogen molecules are combined with hydrogen molecules to make what is called an amino group.

Although the molecular weight is usually high, there is a vast range in both structure and complexity of protein molecules.

Hemoglobin for example, has a molecular weight of about 64,500; myosin, a muscle protein is estimated to have a molecular weight of about 489,000.

On the average, about 20 different amino acids occur in most proteins, the amino acids present, their position in the molecule, and the spatial arrangement of the molecule all determine the proteins and characteristics of the proteins. In turn the function of a protein depends, in large measure, on its structure.

Proteins play a critical role in virtually every physiological and biochemical process in the body.

Protein serves as cell communicators through the action of neurotransmitters. They are also essential for blood clotting, immune system development and formation of milk during lactation.
Proteins and amino acids

What is cysteine?

Cysteine is found extensively in the plant foods human ingest. It is found plentifully in the grasses and thus in the meat of domesticated grazing animals.

Chemically very reactive, cysteine is a sulfur containing amino acid that is unique of its chemistry sulfhydryl group.

Cysteine takes its name from cystine, named after the Greek kustis meaning bladder – cystine was first isolated for kidney stones.

Cysteine, in the presence of mild oxidizing agents, readily dimerizes, that is reacts with another cysteine molecule to form a cysteine molecule.

The sulfur in cysteine molecules plays a crucial role in folding proteins into their correct shapes. For examples sulfur independent proteins are keratin – part of hair, skin and nails and collagen – part of connective tissue like cartilage.

In addition, cysteine is a precursor of methionine and also thiamine, biotin, lipoic acid, coenzymes A and coenzyme M.

The amino acid contains a sulfur group that help to function as antioxidant. Cysteine also can be combined with glutamic acid and glycine in liver cells to form glutathione, which is a principle water soluble antioxidant in cells and the blood.

The human body synthesizes the amino acid cysteine for homocysteine and it is part of human hair, skin and nails as the protein keratin.

Cysteine can be found in red pepper, garlic, onions, broccoli, brussel sprouts, oats and wheat germ. 

It is used by bakers to break up the gluten in flour, thus recuing its stickiness and facilitating the kneading of the dough.

Due to their high reactivity, cysteine and its derivatives such as acetylcysteine are also used in expectorants.
What is cysteine?