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Proteins
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PROTEINS
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Amino acids contain an amino group, a carboxyl group, a carbon and a unique R group
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Polar R groups make the amino acid hydrophilic
Non-polar R groups make the amino acid hydrophobic
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Ionic R groups make the amino acid hydrophilic
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There are 20 different amino acids in polypeptides synthesized on ribosomes.
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“Essential Amino Acids” are those that must be ingested in the diet
(our body can’t make them)
Chart for reading amino acids Amino acid name – abb - Letter• alanine - ala - A • arginine - arg - R ***• asparagine - asn - N • aspartic acid - asp - D • cysteine - cys - C • glutamine - gln - Q • glutamic acid - glu - E • glycine - gly - G • histidine - his - H ***• isoleucine - ile - I • leucine - leu - L • lysine - lys - K • methionine - met - M • phenylalanine - phe - F • proline - pro - P • serine - ser - S • threonine - thr - T • tryptophan - trp - W • tyrosine - tyr - Y • valine - val - V
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• Understandings:
• Amino acids are linked together by condensation to form Polypeptides
02
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• Peptide Bonds join amino acids
• It’s a condensation reaction
• (meaning that H20 is released
• when the bond is formed).
Two amino acids form a • DI-PEPTIDE
• POLYPEPTIDES
• are formed from more
• than two amino acids
• bonded together
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Condensation
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Amino acids can be linked together in any sequence, giving a huge range of possible polypeptides
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• A protein may consist of a single polypeptide
or more than one polypeptide linked together
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The amino acid sequence determines the three - dimensional conformation of a protein.
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Proteins have four levels of organization
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• Primary structure is the amino acid sequence
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The amino acid sequence of polypeptides is coded for by DNA (genes) and is unique for each kind of protein.
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• The amino acid sequence determines how the polypeptide will fold into its 3D shape
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Even a slight change in the amino acid sequence can cause the protein to
malfunction
For example, mis-formed hemoglobin causes sickle cell
disease
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Proteins have four levels of organization
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• Secondary structure results from hydrogen bonding between the oxygen of one amino acid and the hydrogen of another
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The alpha helix is a coiled secondary structure due to a hydrogen bond every
fourth amino acid
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The beta pleated sheet is formed by hydrogen bonds between parallel parts
of the protein
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A single polypeptide may have portions with both types of secondary structure
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Proteins have four levels of organization
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Tertiary structure depends on the interactions among the R group side chains
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Types of interactions
• Hydrophobic interactions: amino acids with nonpolar side chains cluster in the core of the protein, out of contact with water
= charged
= hydrophobic
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Types of interactions
• Hydrogen bonds between polar side chains
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Types of interactions
• Ionic bonds between positively and negatively charged side chains
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Types of interactions• Disulfide bridge (strong covalent bonds)
between sulfur atoms in the amino acid cysteine
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Quaternary structure results from interactions among separate
polypeptide chains.
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The folding of proteins is aided by other proteins, called chaperones
• Act as temporary braces as proteins fold into their final conformation
• Research into chaperones is a
area of research in biology
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Denaturation
• May be due to changes in: – pH– Temperature– Various chemicals
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Denaturation results in disruption of the secondary, tertiary, or quaternary
structure of the protein
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Protein function is lost during denaturation, which is often irreversible
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Living organisms synthesize many different
proteins with a wide range of functions.
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Folded proteins are placed into two general categories
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Fibrous proteins have polypeptide chains organized in long fibers or
sheets• Water insoluble• Very tough physically,
may be stretchy
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Functions of fibrous proteins
• Structural proteins function in support– Insects and spiders
use silk fibers to make cocoons and webs
– Collagen and elastin are used in animal tendons and ligaments
– Keratin is the protein in hairs, horns and feathers
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Functions of fibrous proteins
• Contractile proteins function in movement– Actin and myosin
contract to create the cleavage furrow and to move muscles
– Contractile proteins move cilia and flagella
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Globular proteins have their chains folded into compact, rounded shapes
• Easily water soluble
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Functions of globular proteins
• Storage proteins function in the storage of amino acids– Ovalbumin is the
proteins in egg whites– Casein is the protein in
milk, source of amino acids for baby mammals
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Functions of globular proteins
• Transport proteins function in the movement of other substances
• Haemoglobin, the iron containing protein in blood, transport oxygen from lungs to other parts of the body (C3032H4816O872N780S9Fe4)
• Membrane transport proteins such as channels for potassium and water
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Functions of globular proteins
• Hormone proteins function as cellular messenger molecules that help maintain homeostasis
• Insulin: sends message “allow sugar into cells” (when blood glucose levels are high, cells will transport glucose into the cells for use or storage)
• Glucagon: sends message “we need more sugar in the blood” (when blood glucose is too low, cells will release glucose)
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Functions of globular proteins
• Receptor proteins allow cells to respond to chemical stimuli– Growth factor receptors initiate the signal
transduction pathway when a growth hormone attaches
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Functions of globular proteins
– Cholesterol receptors on the cell membrane allow LDL to be endocytosed into the cell
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Functions of globular proteins
• Protective proteins function as protection against disease– Antibodies combat bacteria and viruses
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Functions of globular proteins
• Enzymes speed up chemical reactions– Amylase and other digestive enzymes
hydrolyze polymers in food
– Catalase converts hydrogen peroxide H2O2 into water and oxygen gas during cellular respiration
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Active Genes
• Depends what type of cell they are
• Number of active genes varies by organism
Common name of the organism Approximate number of genes in the organism’s genome
Yeast (single-celled fungi) 6,000
Drosophila (fruit fly) 14,000
Rice plant 51,000
Laboratory mouse 30,000
Domestic dog 19,000
Humans 20-25,000
Number does not correlate to complexity
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Every individual has a unique proteome.