LECTERE 5 THEME: AMINO ACIDS, PEPTIDES AND PROTEINS. Lecturer: Yevheniya B. Dmukhalska
PLAN • Biological role of amino acidsand proteins • Classification of amino acids. • Structure of amino acids. • Chemical properties of amino acids. • Peptides. • Proteins: levels of structure of proteins. • Compound proteins.
PROTEIN • А protein is in polymer in which the monomer units are aminoacids. Thus the starting point for а discussion of proteins is an understanding of the structures and chemical properties of aminoacids. • The word protein comes from the Greek proteios, which means "of first importance." This derivation alludes to the key role that proteins play in life processes.
Proteins are the most abundant substances in most cells - from 10% to 20% of the cell’s mass. • More than 70-80 % of dry weight of muscles, lungs, kidneys, spleen; 57 % of dry weight of liver, 45 % of dry weight of brain are proteins. The lowest proteins constituting in bones and teeth (20 and 18 % responding). • Contentsofchemicalelementsinproteins: carbon is 51-55 %, oxygen is 21-28 %, nitrogenis 15-18 %, hydrogenis 6-7 %, sulfur is 0.3-2.5 %. Someproteinscontainphosphorusiron, zinc, copperandotherelements- (0.2-2%).
AMINO ACIDS • An amino acid is an organic compound that contains both an amino (–NН3) group and a carboxyl (-СООН) group. The amino acids found in proteins are always α-amino acids.
Classification of amino acids • Nonpolar amino acids contain one amino group, one carboxyl group, and a nonpolar side chain. • Polar neutral amino acids contain one amino group, one carboxyl group, and а side chain that is polar but neutral. • Polar acidic amino acids contain one amino group and two carboxyl groups, the second carboxyl group being part of the side chain. There are two polar acidic amino acids: aspartic acid and glutamic acid. • Polar basic amino acids contain two amino groups and one carboxyl group, the second amino group being part of the side chain. There are three polar basic amino acids: lysine, arginine, and histidine.
The most important aminoacids Monoaminomonocarboxylic aminoacids Glycine Leucine Alanine Isoleucine Valine
The most important aminoacids Diaminomonocarboxylic aminoacids Lysine Ornytine Mnoaminodicarboxylicaminoacids Glutamic acod Aspartic acid Hydroxylaminoacids Threonine Serine
The most important aminoacids Sulphorcontain aminoacids Methionine Cysteine Cystine Aromaticaminoacids Phenyalanine Tyrosine
The most important aminoacids Heterocyclic aminoacids Histidine Tryptophane Proline Hydroxiproline Aminoacids with amide group Arginine
Essential and non-essential amino acids • All of the 20 amino acids are necessary constituents of human protein. Adequate amounts of 11 of the 20 amino acids can be synthesized from carbohydrates and lipids in the body if а source of nitrogen is also available. Because the human body is incapable of producing 9 of these 20 acids, these 9 amino acids, called essential amino acids, must be obtained from food. The human body can synthesize small amounts of some of the essential amino acids, but not enough to meet its needs, especially in the case of growing children. • The 9 essential amino acids for adults are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. (In addition, arginine is essential for children).
Acidity Both an acidic group (-СООН) and а basic group (-NН2) are present on the same carbon in an α-amino acid. The net result is that in neutral solution, amino acid molecules have the structure: А zwitterionis а molecule that has а positive charge on one atom and а negative charge on another atom.
Reaction of amino acids • Reaction with alcohols – esters formation: • Reaction with ammonia – amides formation: • Decarboxylation: Salts are formed:
Deamination • oxidation deamination: • hydrolitic deamination: • intramolecular deamination: • redaction deamination:
Peptide formation • Two amino acids can react in а similar way - the carboxyl group of one amino acid reacts with the amino group of the other amino acid. The products are а molecule of water and а molecule containing the two amino acids linked by an amide bond.
Ninhydrin test. The ninhydrin colour reaction is the most commonly test used for the detection of alpha-amino acids. This is an extremely delicate test, to which proteins, their hydrolytic products, and α-amino acids react. When ninhydrin is added to а protein solution and the mixture is heated to boil, blue to violet colour appears on cooling. The colour is due to the formation of а complex compound.
Qualitative tests for amino acids and protein. XANTHOPROTEIC TEST. On treatment with concentrated nitric acid, certain proteins give yellow colour. This yellow colour is the same that is formed on the skin when the latter comes in contact with the concentrated nitric acid. The test is given only by the proteins having at least one mole of aromaticaminoacid, such as tryptophan, phenylalanine, and tyrosinewhichare actually nitrated during treatment with concentrated nitric acid.
FOLL REACTION. This reaction reveals the sulfur containing amino acids (cysteine, cystine). Treatmentofthesulfurcontainingaminoacidswithsaltofleadandalkaliyieldsablacksediment. ADAMKEVICH REACTION. This reaction detects the amino acid tryptophan containing indol ring. Theadditionoftheconcentratedaceticandsulfuricacidstothesolutionoftryptophanresultsintheformationofred-violetringappearingontheboundaryofdifferentliquids.
Peptides • Short to medium-sized chains of amino acids are known as peptides. А peptide is а sequence of amino acids, of up to 50 units, in which the amino acids are joined together through amide (peptide) bonds. А compound containing two amino acids joined by а peptide bond is specifically called а dipeptide; three amino acids in а chain constitute а tripeptide; and so on. The name oligopeptide is loosely used to refer to peptides with 10 to 20 amino acid residues and polypeptide to larger peptides.
Proteins • Proteins are polypeptides that contain more than 50 amino acid units. The dividing line between а polypeptide and а protein is arbitrary. The important point is that proteins are polymers containing а large number of amino acid units linked by peptide bonds. Polypeptides are shorter chains of amino acids. Some proteins have molecular masses in the millions. Some proteins also contain more than one polypeptide chain.
Function of proteins • Catalysis. Enzymes, the proteins that direct and accelerate thousands of biochemical reactions • Structure. Some proteins function as structural materials that provide protection and support. • Movement. Proteins are involved in all types of cell movement. For example, actin, tubulin, and а variety of other proteins comprise the cytoskeleton. • Defense. А wide variety of proteins have а protective role. Examples found in vertebrates include keratin, the protein found in skin cells that aids in protecting the organism against mechanical and chemical injury. The blood-clotting proteins fibrinogen and thrombin prevent blood loss when blood vessels are damaged. The immuno-globulins (or antibodies) are produced by lymphocytes in response to the invasion of foreign organisms such as bacteria. • Regulation. The binding of а hormone molecule to its target cell results in specific changes in cellular function. Examples of peptide hormones include insulin and glucagon, which regulate blood glucose levels. Growth hormone stimulates cell growth and division. • Transport. Many proteins function as carriers of molecules or ions across membranes or between cells. Examples of membrane proteins include the Na+-К+ ATPaseand the glucose transporter. Other transport proteins include hemoglobin, which carries O2 to the tissues from the lungs, and the lipoproteins, which transport lipids from the liver and intestines to other organs
Primarystructureof а protein • The primary structure of а protein is the sequence of amino acids present in its peptide chain or chains. • The end with the free H3N+ group is called the N-terminal end, and the end with the free СОО- group is called the С-terminal end.
Secondary structure of а protein • The secondary structure of а protein is the arrangement in space of the atoms in the backbone of the protein. Three major types of protein secondary structure are known; the alpha helix, the beta pleated sheet, and the triple helix. The major force responsible for all three types of secondary structure is hydrogen bonding between а carbonyl oxygen atom of а peptide linkage and the hydrogen atom of an amino group (-NH) of another peptide linkage farther along the backbone.
Alpha Helix • The Alpha Helix The alpha helix (α-helix) structure resembles а coiled helical spring, with the coil configuration maintained by hydrogen bonds between N – Н and С= О groups of every fourth amino acid
Beta pleated sheet • The beta pleated sheet (β-pleated sheet) secondary structure involves amino acid chains that are almost completely extended.
Tertiary structure The tertiary structure of а protein is the overall three-dimensional shape that results from the attractive forces between amino acid side chains (R groups) that are widely separated from each other within the chain.
Interactions responsible for tertiary structure hydrogen bonds; hydrophobic attractions; electrostatic attractions (salt bridges); covalent disulfide bonds;
Electrostatic attractions (salt bridges), Hydrogen bonds,
Quaternary structure • Quaternary structure is the highest level of protein organization. It is found only in proteins that have structures involving two or more polypeptide chains that are independent of each other — that is, are not covalently bonded to each other. These multichain proteins are often called oligomeric proteins. The quaternary structure of а protein involves the associations among the separate chains in an oligomeric protein.
The function of hemoglobin in an organism is to transport oxygen. 1 g of hemoglobin absorbs 1.35 ml of oxygen at STP, corresponding to exactly one molecule of О2 per iron.
Globular and fibrous proteins • Onthebasisofstructuralshape, proteinscanbeclassifiedintotwomajortypes: fibrousproteinsandglobularproteins. • А fibrousproteinisаproteinthathasаlong, thin, fibrousshape. Suchproteinsaremadeupoflongrod-shapedorstring-likemoleculesthatcanintertwinewithoneanotherandformstrongfibers. Theyarewater-insolubleandgenerallyhavestructuralfunctionswithinthehumanbody. • А globularproteinisаproteinwhoseoverallshapeisroughlysphericalorglobular. Globularproteinseitherdissolveinwaterorformstablesuspensionsinwater, whichallowsthemtotravelthroughthebloodandotherbodyfluidstositeswheretheiractivityisneeded.
SimpleandConjugatedProteins • Proteins are classified as either simple proteins or conjugated proteins. • А SIMPLE PROTEIN is made up entirely of amino acid residues. • А COMPOUND PROTEIN has other chemical components in addition to amino acids. These additional components, which may be organic or inorganic, are called prosthetic groups.