Proteins

• A protein is a naturally-occurring, unbranched polymer in which the monomer units are amino acids. Most abundant molecules in the cells after water – account for about 15% of a cell’s overall mass.
• Protein has elemental composition which contain Carbon, Hydrogen, Nitrogen, Oxygen, Sulfur, Iron, and Phosphorus.
• The specific definition of a protein is a peptide in which at least 40 amino acid residues are present
• In several proteins with >10,000 amino acid residues are known
• Common proteins contain 400–500 amino acid residues.
• Small proteins contain 40–100 amino acid residues.
• Monomeric : Contains one polypeptide chain.

  Multimeric: Contains 2 or polypeptide chains
• Amino acid is the building block for protein
• Amino acid is an organic compound that contains both an amino (-NH2) and a carboxyl (-COOH) group attached to the same carbon atom.
• The position of carbon atom is Alpha (a).
• The -NH2 group and -COOH group is attached at alpha (a) carbon atom.
• R side chain – vary in size, shape, charge, acidity, functional groups present, hydrogen-bonding ability, and chemical reactivity.
• There are greater than or more than 700 amino acids are known.
• Based on common “R” groups, there are 20 standard amino acids.
• Fill in the blank

  A) Amino group

  B) a-carbon atom

  C) carboxyl group

  D) hydrogen

  E) r side chain

  F) c

  G) cooh

  H) h2n

  
• All amino acids differ from one another by their R-groups. There are 20 common (standard) amino acids
• Standard amino acids are divided into four groups based on the properties of R-groups.
• Non-polar amino acids: R-groups are non-polar

  such amino acids are hydrophobic-water fearing (insoluble in water). And there are 9 out of the 20 standard amino acids are non-polar.
• In Non-polar amino acids, when present in proteins, they are located in the interior of protein where there is no polarity.
• Fill in the blank

  A) glycine

  B) alanine

  C) valine

  D) leucine

  E) isoleucine

  F) proline

  G) phenylalanine

  H) Methionine

  I) tryptophan

  
• Polar amino acids: R-groups are polar.

  Three types: Polar neutral; Polar acidic; and Polar basic.
• Polar-neutral: contains polar but neutral side chains

  – Six amino acids belong to this category
• Polar acidic: Contain carboxyl group as part of the side chains.

  – Two amino acids belong to this category.
• Polar basic: Contain amino group as part of the side chain. – Three amino acids belong to this category
• Fill in the blank

  A) serine

  B) cysteine

  C) threonine

  D) asparagine

  E) glutamine

  F) tyrosine
• fill in the blank

  A) aspartic acid

  B) glutamic acid

  C) histidine

  D) lysine

  E) arginine

  
• Derived amino acids are “nonstandard” amino acids and is usually formed by an enzyme-facilitated reaction on a common amino acid after that amino acid has been incorporated into a protein structure.
• d-carboxyglutamate found in prothrombin
• Essential Amino acid

  -A standard amino acid needed for protein synthesis that must be obtained from dietary sources

  -adequate amounts cannot be synthesized in human body.

  -Nine of the 20 standard amino acids are considered essential
• fill in the blank

  A) arginine

  B) histidine

  C) isoleucine

  D) leucine

  E) lysine

  F) Methionine

  G) phenylalanine

  H) threonine

  I) tryptophan

  J) valine

  
• Some of the first information on the biological value of dietary proteins came from studies on rats. In one series of experiments, young rats were fed diets containing 18% protein in the form of either casein (a milk protein), gliadin (a wheat protein), or zein (a corn protein).

  Results:

  Casein: rats remained healthy and grow normally

  Gliadin: rats maintained their weight but did not grow much

  Zein: rats not only failed to grow but began to lose weight, & eventually died if kept on this diet
• Since casein evidently supplies all the required amino acids in the correct proportions needed for growth, it is called a complete protein.
• A complete protein contains all the essential amino acids in the proper amounts.
• Complementary proteins are incomplete proteins which when served together, complement each other and provide all the essential amino acids.
• incomplete protein is low in one or more of the essential amino acids, usually lysine, tryptophan, or methionine.
• Four different groups are attached to the a-carbon atom in all of the standard amino acids except glycine. In glycine R-group is hydrogen.
• Therefore 19 of the 20 standard amino acids contain a chiral center.
• Molecules with chiral centers exhibit enantiomerism (left- and right- handed forms).
• The amino acids found in nature as well as in proteins are L isomers.

  – Bacteria do have some D-amino acids.

  – With monosaccharides nature favors D-isomers.
• Acid–Base Properties of Amino Acids

  • In pure form amino acids are white crystalline solids.

  • Most amino acids decompose before they melt.

  • Not very soluble in water.