Unit 5, Lesson 8

Discovery of Enzymes

In 1926, American chemist James B. Sumner reported 1st isolation & crystallization of enzyme Enzyme he isolated was urease, which hydrolyzes urea, constituent of urine, into ammonia & carbon dioxide

More Amino Acids

More amino acids may be added to peptide in same fashion to form long chains by condensation polymerization Order in which amino acids of peptide molecule are linked is called amino acid sequence of molecule Amino acid sequence of peptide is conveniently expressed using 3-letter abbreviations for amino acids For example, Asp---Glu---Gly represents peptide containing 3 amino acids; this tripeptide contains aspartic acid, glutamic acid, & glycine, in that order, with free amino group assumed to be on left end (on the Asp) & free carboxyl group on right end (on the Gly) Note that Asp---Glu---Gly is different peptide from Gly---Glu---Asp because order of amino acids is reversed & thus free amino group & free carboxyl group are on different amino acids

Folding Protein Molecules

Protein molecules are folded into relatively stable 3-dimensional shapes Peptide chains may be arranged side by side to form a pleated sheet or may coil into a regular spiral called a helix Irregular folding of chains can also occur; 3-dimensional shape of protein is determined by interactions among amino acids in peptide chains Protein shape is partly maintained by hydrogen bonds between adjacent folded chains; covalent bonds also form between sulfur atoms of cysteine side chains folded near each other; in that way, separate polypeptide chains may be joined into single protein Myoglobin, protein that stores oxygen in muscle cells, has peptide chains that twist into helixes

Coenzymes

Some enzymes can directly catalyze transformation of biological substrates without assistance from other substances; other enzymes need nonprotein coenzymes (also called cofactors) to assist transformation Coenzymes are metal ions or small organic molecules that must be present for enzyme-catalyzed reaction to occur Many water-soluble soluble vitamins (such as B vitamins) are coenzymes; metal ions that act as coenzymes include cations of magnesium, potassium, iron, & zinc Enzyme catalase includes iron(III) ion in its structure; catalase catalyzes breakdown of hydrogen peroxide to water & oxygen

What do the amide bonds between amino acids always involve?

always involve the central amino & central carboxyl groups; side chains are not involved in bonding

peptide

any combination of amino acids in which the amino group of one amino acid is united with the carboxyl group of another amino acid through an amide bond

amino acid

any compound that contains an amino group (---NH2) & a carboxyl group (---COOH) in the same molecule; proteins are made from the 20 naturally occurring amino acids; have a skeleton that consists of a carboxyl group & an amino group, both of which are covalently bonded to a central carbon atom; remaining 2 groups on central carbon atom are hydrogen & an R group that constitutes amino acid side chain

protein

any peptide with more than 100 amino acids; molecule of amino acids has molar mass of 10,000 amu; skin, hair, nails, muscles, & hemoglobin molecules in blood are made of protein; proteins needed for almost all chemical reactions that take place in body

enzymes

proteins that act as biological catalysts; increase rates of chemical reactions in living things; catalyze most of the chemical changes that occur in the cell

What happens in a typical enzymatic reaction?

substrate interacts with side chains of amino acids on enzyme; these interactions cause making & breaking of bonds; substrate molecule must make contact with & bind to enzyme molecule before substrate can be transformed into product

peptide bond

the amide bond between the carboxyl group of one amino acid & the nitrogen in the amino group of the next amino acid in the peptide chain

active site

the place on an enzyme where a substrate binds; usually a pocket or crevice formed by folds in peptide chains of enzyme product; peptide chain of enzyme is folded in unique way to accommodate for substrate here

In theory,

the process of adding amino acids to a peptide chain may continue indefinitely

polypeptide

any peptide with more than 10 amino acids

How many amino acid sequences are possible for a protein of 100 amino acids containing a combination of the 20 different amino acids?

as many as 20^100 amino acid sequences

What do differences in the chemical & physiological properties of peptides & proteins result from?

differences in the amino acid sequence

substrates

molecules on which an enzyme acts

Differences in Properties Between Amino Acids

Chemical nature of side-chain group accounts for differences in properties among 20 amino acids In some amino acids, side chains are nonpolar aliphatic or aromatic hydrocarbons; in other amino acids, side chains are neutral but polar; in others, side chains are acidic or basic Because central carbon of amino acids is asymmetric, these compounds can exist as optical isomers; optical isomers may be right- or left-handed, but nearly all amino acids found in nature are of the left-handed (L) form

Free Groups

Free amino group remains at one end of resulting peptide Convention is to write formula of peptide so that free amino group is at left end There is also free carboxyl group that appears at right end

Properties of Enzymes

Have 3 properties of true catalysts Able to promote reactions; unchanged by reaction they catalyze, & do not change normal equilibrium position of chemical system Same amount of product is equally formed whether or not enzyme is present, but few reactions ever reach equilibrium; products tend to convert rapidly to another substance in subsequent enzyme-catalyzed reaction

Amino Acid Abbreviations

Alanine - Ala Arginine - Arg Asparagine - Asn Aspartic acid - Asp Cysteine - Cys Glutamine - Gln Glutamic acid - Glu Glycine - Gly Histidine - His Isoleucine - Ile Leucine - Leu Lysine - lys Methionine - Met Phenylalanine - Phe Proline - Pro Serine - Ser Threonine - Thr Tryptophan - Trp Tyrosine - Tyr Valine - Val

One Substrate

Because active site of each enzyme has distinctive shape, only 1 specific substrate molecule can fit into enzyme, in same way that only 1 key will fit into certain lock Each enzyme catalyzes only 1 chemical reaction, with only 1 substrate Enzyme-substrate complex is formed when enzyme molecule & substrate molecule are joined

Efficiency of Enzymes

To see efficiency of enzymes, consider enzyme called carbonic anhydrase; catalyzes reversible breakdown of carbonic acid to carbon dioxide & water 1 molecule of carbonic anhydrase can catalyze breakdown of 36 million molecules of carbonic acid in 1 minute