Introduction
Proteins are the most abundant type of organic molecule in living systems
Proteins carry out various functions inside the body:
- Structural proteins – Collagen, and elastin which is predominant in bones, vascular systems, and matrix.
- Dynamic functions – Enzymes, hormones, blood clotting factors, and immunoglobulins.
Proteins are polymers of amino acids
Amino acids
Amino acids are composed of two functional groups which are amino group and a carboxyl group
Classification
- Aliphatic side-chain – Glycine, alanine, valine, leucine, isoleucine
- Hydroxyl groups – Serine, threonine, tyrosine
- Acidic and amides – Aspartic acid, asparagine, glutamic acid, and glutamine
- Basic – Arginine, lysine, and Histidine
- Aromatic – Phenylalanine, tyrosine, tryptophan
- Imino Acids – Proline
Essential and non-essential amino acids
- Essential amino acids are not synthesized by the body. Arginine, Valine, Isoleucine, Histidine, Lysine, methionine, phenylalanine
- Non-essential amino acids – The body can synthesize these amino acids and hence not required in the diet. Glycine, Alanine, Proline, Aspartate, Cysteine, Asparagine.
Metabolic fate of amino acids
- Glycogenic amino acids – Amino acids = Glucose/glycogen
- Ketogenic amino acids – Amino acids = fats
Properties of Amino Acids
- Soluble in water
- High melting point
- Some amino acids have a sweet taste, and others are tasteless
- All amino acids except glycine have optical isomers
- Amino acids have both carboxyl group and amino group, acting as proton acceptor and proton donor (Ampholytes).
Isoelectric pH
This is the pH at which the molecule contains both positive and negative ionic groups and carries no net charge
Protein structure
- Primary structure – A linear sequence of amino acids in a polypeptide
- Secondary structure – The spatial arrangement of protein that is formed by the twisting of polypeptides
- Tertiary structure – The three-dimensional structure of the protein
- Quarternary structure – This is composed of two or more polypeptides
A peptide bond is formed by an amino group and a carboxyl group
Properties of proteins
- Forms colloidal solution in water
- Its molecular weight is dependent on the number of amino acids
- Protein shape may be globular, oval, fibrous, or elongated
- Protein precipitation may occur by dehydration and neutralization of polar groups
Protein denaturation may occur as a result of:
- Physical agents – Heat, violent shaking, X-rays, UV radiation
- Chemical agents – Acids, alkalines, Organic solvents, salts of heavy metals, urea or salicylate
Protein denaturation results to:
- Loss of protein helical structure
- Loss of biological activity
- The protein becomes easily digested
- The denaturation is irreversible
- The protein cannot be crystallized
Classes of proteins
- Single proteins – Formed by amino acids only
- Conjugated amino acids – Non-protein moiety + amino acid
- Derived proteins – Denatured products of single and conjugated proteins.
Revision
Proteins
-Composed of carbon, hydrogen, oxygen and nitrogen
-Arranged as strands of amino acids
Amino Acids
-The building blocks of proteins. Each amino acid has an amine group at one end and an acid group at the other, and a distinctive side chain. There are about 20 different amino acids that make up most of the proteins.
Amine Group
the nitrogen containing portion of an amino acid
Side chain
the unique chemical structure attached to the backbone of each amino acid that differentiates one amino acid from another
Essential amino acids
-Amino acids that either cannot be synthesized at all by the body or cannot by synthesized in amounts sufficient to meet physiological need, they are also called indispensable amino acids
Conditionally essential amino acids
-An amino acid that is normally non-essential but must be supplied by the diet in special circumstances when the needs for it exceeds the body’s ability to produce it
Formation of proteins
-Amino acids are hooked together with peptide bonds to form protein chains, these chains can be anywhere from several dozen to 300 amino acids long
-The protein chains coil and can either be functional as it is, or it may need to join with other proteins, carbs, vitamins, or minerals to be functional
-The body also has the ability to breakdown protein molecules and reuse the amino acids to form new proteins
Denaturation of Proteins
-Denaturation is the first step in the destruction of a protein. Proteins can be denatured by heat, radiation, alcohol, acids, bases or the salts in heavy metals
-These agents can disrupt a proteins folded structure making it unable to function normally in the body, however denaturation is helpful in the digestion of proteins in the body.
Variety of Proteins
-Each type of protein had its own distinctive sequence of amino acids
-Some common proteins:
-collagen- a type of body protein from which connective tissues such as scars, tendons, and bones are made
-enzymes- proteins that facilitate chemical reactions without being changed in the process: they are essential protein catalysts
-hemoglobin-the globular protein of red blood cells, whose iron atoms carry oxygen around the body via the bloodsteam
Protein Digestion
In stomach: acid denatures the protein stands, and an enzyme cleaves amino acid strands into polypeptides and a few amino acids
In small intestine: enzymes from the pancreas and intestine split peptide stands into tri/dipeptides and amino acids
In bloodstream: the bloodstream transports amino acids to all the body’s cells
Roles of Body Proteins
-Structure and movement: 40% of the body’s protein exists in muscle tissue, these proteins allow the body to move and play key roles in muscle structure
-Building antibodies: large proteins found in the blood which are produced by the immune systems in response to an invasion of the body by a foreign substance (antigens). Antibodies combine with and inactivate antigens
-Transporting Substances: Many proteins specialize in transporting substances throughout the body such as lipids, vitamins, minerals, and oxygen. A common example of a transporting protein is hemoglobin which transports oxygen from the lungs to the cells.
-Blood Clotting: To prevent dangerous levels of blood loss, special blood proteins form a stringy net that traps blood cells to form a clot.
Need of Quality Protein:
DRI recommendations
-Women: 46 grams/day
-Men: 56 grams/day
-10-30 % of calories from protein
Need of Quality Protein:
high quality protein
-Dietary proteins containing all the essential amino acids in relatively the same amounts that humans require
Need of Quality Protein:
limiting amino acid
-An essential amino acid that is present in dietary protein in an insufficient amount, thereby limiting the body’s ability to build protein
Need of Quality Protein:
complementary proteins or mutual supplementation
-Two or more proteins whose amino acid assortments complement each other in such a way that the essential amino acids missing from on are supplied by the other
Protein Deficiency:
Protein-energy malnutrition (PEM)
-The world’s most widespread malnutrition problem, including both marasmus and kwashiorkor and states in which they overlap
-Also called protein-calorie malnutrition (PCM)
-PEM is essentially starvation
Marasmus
A form of PEM related to protein malnutrition and infections, with a set of recognizable symptoms, such as edema
Kawshiorkor
A form of PEM related to protein malnutrition and infections, with a set of recognizable symptoms, such as edema
Protein is Excess
-Overconsumption of protein-rich foods may pose health risks
-A build-up of the amino acid homocysteine, produced as an intermediate compound during amino acid metabolism, is associated with deficiencies of the B vitamins and may increase the risk of diseases
-Increased intakes of purified protein may be associated with causing or worsening adult bone loss and increasing the effects of osteoporosis, a disease characterized by the weakening of the bones
