Amino Acids and Proteins Larry Scheffler Lincoln High School Portland OR 1

Amino Acids Amino acids have both a carboxyl group -COOH an amino group -NH2 in the same molecule..  2

Amino Acid Structure The general formula of an amino acid is shown here The group designated by R is usually a carbon chain but other structures are also possible 3

Amino Acid Structure Amino acids may be characterized as a, b , or g amino acids depending on the location of the amino group in the carbon chain. a are on the carbon adjacent to the carboxyl group. b are on the 2nd carbon g on the 3rd carbon from the carboxyl group 4

Amino Acids - Proteins Amino acids are the building blocks of proteins. Proteins are natural polymers of successive amino acids There are 20 different amino acids that make up human proteins 5

a- amino acids Amino acids found in proteins are a- amino acids. The amino group is always found on the carbon adjacent to the carboxyl group 6

Amino Acid Functions Amino acids are the building blocks of proteins Some amino acids and their derivatives function as neurotransmitters and other regulators Examples Include L-dopamine Epinephrine Thyroxine Histidine 7

Amino Acids and Proteins Amino acids forming proteins may be characterized as Acidic, Basic, or neutral depending on the character of the side chain attached. 8

Acidic Amino Acids There are two acidic amino acids. There are two carboxyl groups and only one amino group per molecule (asp) (glu) 9

Basic Amino Acids I These amino acids are basic. They have more amino groups than carboxyl groups 10

Basic Amino Acids II These amino acids are also basic. They have more amino groups than carboxyl groups 11

Neutral Amino Acids I These amino Acids are considered neutral. There is one carboxyl group per amino group (ala) (gly) ((leu) 12

Neutral Amino Acids II (Ser) (Tyr) (Val) (Trp) (Cys) (Met) 13

Neutral Amino Acids III (Ile) (Thr) (Asp) (Phe) (Gln) (Pro) 14

Amino Acids and Optical Isomers Except for glycine, all amino acids have a chiral carbon atom. Therefore they can have optical isomers The amino acids found in proteins are all levarotatory or L forms. 15

Amino Acids are Amphoteric Amino acids are amphoteric. They are capable of behaving as both an acid and a base, since they have both a proton donor group and a proton acceptor group. In neutral aqueous solutions the proton typically migrates from the carboxyl group to the amino group, leaving an ion with both a (+) and a (-) charge. 16

The Zwitterion This dipolar ion form is known as a Zwitterion. 17

Essential Amino Acids Of the 20 amino acids that make up proteins 10 of them can be synthesized by the human body The other 10 amino acids must be acquired from food sources. These amino acids are known as essential amino acids 18

Essential Amino Acids Essential amino acids Arginine Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine Non-Essential amino acids Alanine (from pyruvic acid) Asparagine (from aspartic acid) Aspartic Acid (from oxaloacetic acid) Cysteine Glutamic Acid (from oxoglutaric acid) Glutamine (from glutamic acid) Glycine (from serine and threonine) Proline (from glutamic acid) Serine (from glucose) Tyrosine (from phenylalanine) 19

Essential Amino Acids Complete protein Incomplete protein Contains all 10 essential amino acids Proteins derived from animal sources are complete proteins Beans contain some complete protein as well Incomplete protein Lack one of more of the essential amino acids Most vegetable proteins are incomplete proteins Beans are an exception to this generalizations 20

Peptide Bond When two amino acids combine, there is a formation of an amide and a loss of a water molecule + H2O 21

Proteins- Levels of Structure Amino acids can undergo condensation reactions in any order, thus making it possible to form large numbers of proteins. Structurally, proteins can be described in four ways. Primary Secondary Tertiary Quaternary structure. 22

Primary Structure The primary structure of a protein is defined by the sequence of amino acids, which form the protein. This sequence is determined by the base pair sequence in the DNA used to create it. The sequence for bovine insulin is shown below 23

Secondary Structure The secondary structure describes the way that the chain of amino acids folds itself due to intramolecular hydrogen bonding Two common secondary structures are the a-Helix  and the b- sheet  24

Tertiary Structure The tertiary structure maintains the three dimensional shape of the protein. The amino acid chain (in the helical, pleated or random coil form) links itself in places to form the unique twisted or folded shape of the protein. 25

Tertiary Structure There are four ways in which parts of the amino acid chains interact to stabilize its tertiary shape.. They include: -- Covalent bonding, for example disulfide bridges formed when two cysteine molecules combine in which the –SH groups are oxidized: -- Hydrogen bonding between polar groups on the side chain. -- Salt bridges (ionic bonds) formed between –NH2 and –COOH groups -- Hydrophobic interactions. 26

Quaternary Structure Many proteins are not single strands The diagram below shows the quaternary structure of an enzyme having four interwoven amino acid strands 27

Denaturing Proteins The natural or native structures of proteins may be altered, and their biological activity changed or destroyed by treatment that does not disrupt the primary structure. Following denaturation, some proteins will return to their native structures under proper conditions; but extreme conditions, such as strong heating, usually cause irreversible change. 28

Denaturing Proteins Heat Ultraviolet Radiation Strong Acids or Bases hydrogen bonds are broken by increased translational and vibrational energy.(coagulation of egg white albumin on frying.) Similar to heat (sunburn) salt formation; disruption of hydrogen bonds. (skin blisters and burns, protein precipitation.) competition for hydrogen bonds. (precipitation of soluble proteins.) (e.g. ethanol & acetone) change in dielectric constant and hydration of ionic groups. (disinfectant action and precipitation of protein.) shearing of hydrogen bonds. (beating egg white albumin into a meringue.) Heat Ultraviolet Radiation Strong Acids or Bases Urea Some Organic Solvents Agitation 29

Sickle Cell Anemia A small change in the sequence of the primary structure can have a significant impact on protein structure In sickle cell anemia a glutamic acid is replaced by a valine in the amino acid sequence 30

Ninhydrin Reaction Triketohydrindene hydrate, commonly known as ninhydrin, reacts with amino acids to form a purple colored imino derivative, This derivative forms a useful test for amino acids, most of which are colorless. 31

Protein Tests: Biuret Biuret reagent is a light blue solution containing Cu2+ ion in an alkaline solution. Biuret turns purple when mixed with a solution containing protein. The purple color is formed when copper ions in the biuret reagent react with the peptide bonds of the polypeptide chains to form a complex. 32

Xanthroprotic Test Concentrated Nitric acid will form a yellow complex with tryptophan and Tyrosine side chains in proteins 33

Disulfide Bridge Test Disulfide bridges will react with Pb2+ ion from lead acetate in an acidfied solution. A black precipitate indicates the presence of disulfide-bonded cysteine in proteins. 34