UNIT 2 Amino acids and Proteins

UNIT 2 Amino acids and Proteins

Significance of Proteins 1. Keep the cells and tissues growing, renewing and mending 2. Take part in some kinds of important physiological activities 3. Oxidation and supply energy

Structural Movement Transport Storage Hormone Protection Enzymes Collagen; bones, tendons, cartilage Keratin; hair, skin, wool, nails, feathers Myosin & Actin; muscle contractions Hemoglobin; transports O 2 Lipoproteins; transports lipids Casein; in milk. Albumin; in eggs Insulin; regulates blood glucose Growth hormone; regulates growth Immunoglobulins; stimulate immunity Snake venom; plant toxins; Sucrase; catalyzes sucrose hydrolysis Pepsin; catalyzes protein hydrolysis

Amino acids Are the building blocks of proteins. Contain a carboxylic acid group and an amino group on the alpha (α) carbon. Are ionized in solution. Each contain a different side group (R). R COO H H R + H 2 N C COOH H 3 N C ionized form

H + H 3 N C COO H glycine CH 3 + H 3 N C COO H alanine

Amino acids are classified as Nonpolar (hydrophobic) with hydrocarbon side chains. Polar (hydrophilic) with polar or ionic side chains. Acidic (hydrophilic) with acidic side chains. Basic (hydrophilic) with NH 2 side chains. Nonpolar Polar Acidic Basic

Essential amino acids Must be obtained from the diet. Are the ten amino acids not synthesized by the body. Are in meat and diary products. Are missing (one or more) in grains and vegetables. TABLE 19.3

Basic Amino Acids

Amino acids Are chiral except for glycine. Have Fischer projections that are stereoisomers. That are L are used in proteins. H 2 N COOH CH 3 H COOH H NH 2 CH 3 H 2 N COOH H CH 2 SH COOH H NH 2 CH 2 SH L alanine D alanine L cysteine D cysteine

A zwitterion Has charged NH 3+ and COO groups. Forms when both the NH 2 and the COOH groups in an amino acid ionize in water. Has equal + and charges at the isoelectric point (pi). O O + NH 2 CH 2 C OH H 3 N CH 2 C O Glycine Zwitterion of glycine

In solutions more basic than the pi, The NH 3+ in the amino acid donates a proton. + OH H 3 N CH 2 COO H 2 N CH 2 COO Zwitterion at pi Charge: 0 Negative ion ph > pi Charge: 1

In solutions more acidic than the pi, The COO in the amino acid accepts a proton. + H + + H 3 N CH 2 COO H 3 N CH 2 COOH Zwitterion Positive ion at pi ph< pi Charge: 0 Charge: 1+

H + + + OH H 3 N CH 2 COOH H 3 N CH 2 COO H 2 N CH 2 COO positive ion zwitterion negative ion (at low ph) (at ph) (at high ph)

A peptide bond Is an amide bond. Forms between the carboxyl group of one amino acid and the amino group of the next amino acid. O CH 3 O + + H 3 N CH 2 C O + H 3 N CH C O O H CH 3 O + H 3 N CH 2 C N CH C O + H 2 O peptide bond

The primary structure of a protein is The particular sequence of amino acids. The backbone of a peptide chain or protein. CH 3 CH 3 CH 3 CH SH CH 2 S CH 3 O CH O CH 2 O CH 2 O H 3 N CH C N CH C N CH C N CH C O - H H H Ala Leu Cys Met

The secondary structures of proteins indicate the three dimensional spatial arrangements of the polypeptide chains. An alpha helix has A coiled shape held in place by hydrogen bonds between the amide groups and the carbonyl groups of the amino acids along the chain. Hydrogen bonds between the H of a N H group and the O of C=O of the fourth amino acid down the chain.

A beta pleated sheet is a secondary structure that Consists of polypeptide chains arranged side by side. Has hydrogen bonds between chains. Has R groups above and below the sheet. Is typical of fibrous proteins such as silk.

A triple helix Consists of three alpha helix chains woven together. Contains large amounts glycine, proline, hydroxy proline, and hydroxylysine that contain OH groups for hydrogen bonding. Is found in collagen, connective tissue, skin, tendons, and cartilage.

The tertiary structure of a protein Gives a specific three dimensional shape to the polypeptide chain. Involves interactions and cross links between different parts of the peptide chain. Is stabilized by Hydrophobic and hydrophilic interactions. Salt bridges. Hydrogen bonds. Disulfide bonds.

Globular proteins Have compact, spherical shapes. Carry out synthesis, transport, and metabolism in the cells. Such as myoglobin store and transport oxygen in muscle. Myoglobin

Fibrous proteins Consist of long, fiber like shapes. Such as alpha keratins make up hair, wool, skin, and nails. Such as feathers contain beta keratins with large amounts of beta pleated sheet structures.

The quaternary structure Is the combination of two or more tertiary units. Is stabilized by the same interactions found in tertiary structures. Of hemoglobin consists of two alpha chains and two beta chains. The heme group in each subunit picks up oxygen for transport in the blood to the tissues. hemoglobin

Protein hydrolysis Splits the peptide bonds to give smaller peptides and amino acids. Occurs in the digestion of proteins. Occurs in cells when amino acids are needed to synthesize new proteins and repair tissues.

In the lab, the hydrolysis of a peptide requires acid or base, water and heat. In the body, enzymes catalyze the hydrolysis of proteins. OH + CH 3 O H 3 N CH C N H CH 2 CH O C OH heat, H 2 O, H + CH 3 O + H 3 N CH COH + + H 3 N OH CH 2 CH O C OH

Denaturation involves The disruption of bonds in the secondary, tertiary and quaternary protein structures. Heat and organic compounds that break apart H bonds and disrupt hydrophobic interactions. Acids and bases that break H bonds between polar R groups and disrupt ionic bonds. Heavy metal ions that react with S S bonds to form solids. Agitation such as whipping that stretches peptide chains until bonds break.

Denaturation of protein occurs when An egg is cooked. The skin is wiped with alcohol. Heat is used to cauterize blood vessels. Instruments are sterilized in autoclaves.

Transamination of Amino acids Transamination is the process by which an amino group, usually from glutamate, is transferred to an α keto acid, with formation of the corresponding amino acid plus α ketoglutarate. H NH 3 C COO O + C COO Amino transferases O C COO + H NH 3 C COO R 1 R 2 R 1 R 2 amino acid-1 keto acid-1 keto acid-2 amino acid-2

Deamination of Amino acids COOH CHNH 2 NAD + NADH+H + COOH C NH COOH H 2 O NH 3 C O (CH 2 ) 2 COOH L-Glu L-Glu Dehydrogenase (CH 2 ) 2 COOH (CH 2 ) 2 COOH Á-ketoglutarate

1. Sources: Sources and Outlet of NH 3 ⑴ Endogenous sources: 1 Deamination of AAs main source 2 Catabolism of other nitrogen containing compounds 3 Kidney secretion (Gln)

Alanine Glucose Cycle protein muscle blood liver amino acid NH 3 G G G Glu pyruvate pyruvate Glu NAD + + H 2 O Á-keto acid Ala Ala Ala Á-keto acid NADH + H + + NH 3 urea

Transportation of NH 3 by Gln ATP ADP + Pi COOH (CH 2 ) 2 CHNH 2 COOH Glu + NH 3 Gln synthetase Glutaminase CONH 2 (CH 2 ) 2 CHNH 2 COOH Gln H 2 O

Formation of Urea 1. Site: liver (mitochondria and cytosol) 2. Process --------- ornithine cycle urea ornithine NH 3 + CO 2 H 2 O arginase H 2 O Arg citrulline H 2 O NH 3

Metabolism of amino acids Decarboxylation of amino acids Metabolism of one carbon unit Metabolism of sulfur containing AAs Metabolism of aromatic AAs Metabolism of branched chain AAs

Metabolism of Sulphur Containing amino acids Met S CH 3 CH 2 FH 4 CH 2 COOH N 5 -CH 3 FH 4 NH 2 SH CH 2 2 CH NH 2 COOH homocysteine ATP Met synthase VB 12 PPi+Pi adenosyl transferase A H 2 O A S CH 3 CH 2 2 CH NH 2 COOH A SH RH methyl transferase RCH 3 SAM CH 2 2 CH NH 2 COOH S-adenosyl homocysteine

Metabolism of Cysteine and Cystine SH CH 2 CH NH 2 COOH cysteine SH CH 2 + CH NH 2 COOH cysteine 2H 2H S S CH 2 CH 2 CH NH 2 CH NH 2 COOH COOH cystine

Metabolism of Aromatic amino acids NADP + NADPH+H + CH 2 CHNH 2 COOH tetrahydrobiopterin dihydrobiopterin CH 2 CHNH 2 COOH + O 2 Phe hydroxylase + H 2 O Phe OH Tyr

Metabolism of Tryptophan N H Trp CH 2 CHNH 2 COOH O 2 O CCHNH 2 COOH NHCHO N-formyl kynurenine N 10 -CHOFH 4 ADP+Pi FH 2 +ATP N 10 -CHOFH 4 synthetase HCOOH O H 2 O CCHNH 2 COOH NH 2 kynurenine

Metabolism of Tyrosine CH 2 CHNH 2 COOH CH 2 CHNH 2 COOH CH 2 CH 2 NH 2 CO 2 Tyr OH Tyr transaminase HO OH dopa HO OH dopamine O CH 2 CCOOH CH 2 CHNH 2 COOH OH CH 2 CH 2 NH 2 OH hydroxyphenylpyruvate O O dopa quinone HO OH norepinephrine OH SAM CH 2 COOH NH OH OH homogentisate fumarate + acetoacetate O O melanin indole-5,6- quinone HO CH 2 CH 2 NHCH 3 OH epinephrine

Summary of Metabolism