Amino Acids and Proteins B.2

Properties of 2-amino acids (B.2.2)

• Zwitterion (dipolar) – amino acids contain both acidic and basic groups

in the same molecule • therefore, are amphoteric in nature (capable of

behaving as acids or bases)– amines can accept a proton– carboxylic acids can donate a proton

• Buffers– buffer solution • the pH of the solution is "resistant" to small additions of

either a strong acid or strong base• used as a means of keeping pH at a nearly constant value

– the amphoteric nature of amino acids makes it possible for them to act as buffers in aqueous solutions • when a strong acid, H+, is added to an amino acid, the

zwitterion accepts the proton, thus minimizing the effect of the acid added • if a strong base OH- is added, the zwitterion donates H+ to

neutralize the base to form water

• Isoelectric point– the isoelectric point is the pH value at which the

negative and positive charges are equal• this is unique for each a.a. at a certain pH

– used to separate proteins in electrophoresis

Condensation Reactions (B.2.3)

• amino acids will link together to form proteins• enzymes are necessary!• the link is between carboxyl group on one a.a. and

the amino group on the other• water is formed and remaining N and C link

together with a peptide bond• peptide bonds YouTube (1:14)

Primary Structure • determined by the number, kind, and

order of a.a. in the polypeptide. • held together by simple peptide bonds.

Proteins have a complex structure which can be explained by defining four levels of structure (B.2.4)

Secondary Structure

Two types of spontaneously, regular, repeating structures as the polypeptide is made

1. alpha helix – a coil or zigzag shape that results from the hydrogen bonds along the strand

2. beta pleated sheets – back and forth folding of polypeptides because of hydrogen bonds between adjacent polypeptides or in the same strand

Tertiary Structure • highly specific looping and folding of the

polypeptide because of the following interactions between their R-groups:– covalent bonding-- disulfide bridges formed when

two cysteine molecules combine due to sulfur in their R groups

– hydrogen bonding-- between polar groups on the side chain

– ionic attractions-- formed between polar side groups

– van der Waal’s attractions-- between non-polar side groups

• this tertiary level is the final level of organization for proteins containing only a single polypeptide chain

Quaternary Structure

• linkage of two or more polypeptides to form a single protein in precise ratios and with a precise 3-D configuration.

• some proteins have a prosthetic group (a non-peptide)– these proteins are called conjugated proteins• ex. hemoglobin

• John Kyrk folding• Protein folding

Quaternary Structure example

Protein Functions (B.2.6)

• Structure – fibrous proteins provide structure and strength (muscle, cartilage, skin,

bones, hair)

• Transport– hemoglobin in the red blood cells is vital in carrying oxygen

• Hormones – have a regulatory effect on specific cells/organs in the body

• Immunoproteins – play a key role in the fight against infection (antibodies)

• Energy Storage – play an important role in the human body as energy storage

• Enzymes– catalyze biochemical reactions (1000’s) within the body

Analysis of Proteins (B.2.5)

• there are various analytical techniques that can be used to identify proteins and amino acids

• main two are: 1. paper chromatography2. electrophoresis

Paper Chromatography• break peptide bonds in the protein and obtain

constituent a.a.– (use 6 M HCl, 110°C)

• place sample spot on paper and set paper in solvent

• amino acids separate based on polarity

• calculate Rf value and compare to amino acids with know values

solventbymoveddistance

spotbymoveddistanceR f

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Electrophoresis• this technique separates charged molecules

based on their ability to migrate when an electric field is applied to the system sample is placed in a gel and electricity applied

• different a.a. will move at different rates towards a (+) or (-) electrode

• will stop at different distances– this is its isoelectric point where a particular a.a.

will not move as its charges are balanced– distances can then be compared with known

values for identification