FOSC201 - Proteins

Proteinogenic amino acids are

amino acids specifically used by organisms to build proteins

Structure of amino acids

• amino group

• Carboxyl group

• central carbon

• side chain/R

• H atom

Central carbon effects on amino acid properties

• rotation of polarised light

  • D right

  • L left

• physical effect on 3D distribution of the 4 groups

Effect of side chain on amino acid properties

Solubility

Chemical reactivity

H-bonding potential

Net charge

Amino acids more soluble at…temperature

higher

Negatively charged amino acids more soluble at … pH

high

Hydrophobicity

the excess of energy required to transfer one mole of solute from an organic solvent to water

• greater G value the more hydrophobic

• Lower G value more hydrophilic

pH describes the

solution/environment

pKa describes the

molecule

pH > pKa

solution takes protons

• acidic group deprotonates

pH < pKa

molecule takes protons

• basic group protonated

pH close to pI

equal mix of protonated and deprotonated molecules

• net zero charge

HPLC separates amino acids based on

Hydrophobicity

4 protein structures

Primary

Secondary

Tertiary

Quaternary

A peptide is…

a sequence of amino acids covalently bound via carboxyl and amino groups of amino acids

• dehydration reaction (broken by hydrolysis)

• amino group at start

• carboxyl group at end

Things that catalyse hydrolysis/break down of peptides

Heat

Acid

Alkali

Metal ions

Enzymes

R-groups of amino acid determine the way the …. will bend

peptide backbone

Secondary structure is the

(bonds)

3D organisation of relatively close amino acids in the peptide chain

• hydrogen bonds

2 types of secondary protein structure

a - helix

• MALEK (amino acids)

B - sheets

a- helix back bone stabilised by

hydrogen bonds between carboxyl oxygen and amino nitrogen on each turn

B sheets can be

Parallel or antiparallel

Tertiary structure determined by

interactions and bonds of side chains

Tertiary structure arranges…to the inside and … on the surface

non-polar amino acids on inside and polar amino acids on surface

Tertiary structure stabilised by

Covalent disulfide bonds

Hydrophobic interactions

Ionic interactions

Hydrogen bonding

Dipole-dipole interactions

Quaternary structure

Combo of fully folded proteins aggregating in a controlled manner

• same or different sub units

• dimer - 2 subunits

• tetramer - 2 different subunits

Food proteins must be

Easily digestible

Non-toxic

nutritionally adequate

functionally useful

Sustainably sourced

Low potential for allergy

Traditional sources of protein

Milk

Meat

Eggs
Cereals

Legumes

Oilseeds

Tubers

Non-traditional sources of protein

TVP

Single cell proteins

Edible insects

Recommended daily intake of protein

0.8g/kg body weight - average adult

1g/kg body weight - pregnant and lactating women, babies

Kwashiorkor

Abscence of protein in diet

• peripheral and pitting edema

• swelling of gut

• thinning of hair

• breakdown of lean tissue and muscle mass

Protein quality assessed by

scoring theoretical adequacy of essential amino acids

Old methods for assessing protein quality

Nitrogen balance

• nitrogen intake - nitrogen loss

Net protein utilisation

• ratio of amino acid converted to proteins to the ratio of amino acids supplied

9 essential amino acids

Histidine

Isoleucine

Leucine

Lysine

Mehtionine

Phenylalanine

Threonine

Tryptophan

Valine

Current methods for assessing protein quality

PDCAAS

• replaced by DIAAS - amino acids absorbed

Antinutritional factors effecting protein digestibility

Indigenous compounds

• Gluconsinolates

• Trypsin inhibitors

• tannins

compounds produced during processing

• oxidised sulfur amino acids

• maillard compounds

Measuring protein content done in 3 ways

Spectrophotometric/colourmetric

Hydrolysis with quantitation of nitrogen (more accurate)

Hydrolysis with quantitation of amino acids (most accurate)

Examples of spectrophotometric/colorimetric

Biuret

Lowry

Bradford

Turbimetric

Examples of hydrolysis with nitrogen quanititation

Kjeldahl

Dumas

Kjeldahl method - food is digested with…

strong acid so releases nitrogen

Conversion factor used 6.25

adv

• high precision and reproducibility

disadv

• doesn’t measure true proteins

• different conversion factors

• acid and high heat hazard

Dumas method

Food is combusted to release nitrogen

Adv

• faster than Kjeldahl

• easy

Disadv

• high initial cost

• does not measure true protein

• different conversion factors needed

Direct UV spectroscopy

Protein conc measured at 280nm

adv

• simple

• fast

Disadv

• nucleic acids interfere

Biuret method

Peptide bonds react with Cu2+ to give purple colour

• low sensitivity

Lowry method

Biuret reagent combined with Folin ciocalteau phenol

Blueish colour read between 500nm and 750nm

• better sensitivity than biuret

Bradford method

reacted with coomassie Brilliant blue G-250

• shift from brown (465nm) to blue (595nm)

Adv

• sensitive

Disadv

• affected by detergents

What are functional properties

Anything that affects proteins use on the plate

• amino acid comp

• primary sequence of amino acids

• secondary structural elements

• tertiary structure

• charge distribution

• intra and intermolecular bonding

Classes of functional properties

Hydration

Surface active properties

Rheological properties

Sensory properties

Biological

Hydration involves

Hydrophilic properties of amino acids

Hydration - Adding salts makes protein have better

solubility - prevent protein and protein interactions (aggregation)

Hydration - solubility is… dependent

pH

Surface properties involve

hydrophilic and hydrophobic properties

Surface properties examples

Emulsification - stabilize water and oil

Fat binding

Flavour binding

Foaming - stable films entrap gas (partial denaturation needed

Rheological properties involve

Protein - protein interactions

Examples of rheological properties

Gelation

Cohesion - adhesion (partial denaturation and entanglement

Elasticity

Examples of sensory properties

Texturisation (globular to fibrous structure)

Flavour binding

Colour - modified by binding and dispersion of colour compounds by protein

Proteins in egg whites

Ovalbumin

Conalbumin

Lysozyme

Ovomucin

Functional properties proteins in egg whites create

Gelation

Emulsification

Foaming

Water binding

Heat coagulation

Factors that influence functional properties (3)

Intrinsic

Environmental

Process treatments

Intrinsic factors effecting functional properties

Composition of protein

Conformation of protein

mono/multi component

homo/heterogeneity

Environmental factors effecting functional properties

Water

Iona

pH

Oxidising/reducing agents'

lipids, flavours, sugars

Process treatments effecting functional properties

heating

reducing/oxidising agents

Drying

physical modification

chemical modification

Gelation is the process of

forming a gel/polypeptide network that traps water

• heating to induce partial denaturation so bonding zones are increased

• counterbalance forces of attraction and repulsion

• Syneresis = undesirable in gels (expulsion of fluid on surface)

• collagen forms strong gels

Examples of protein gels

Gelatin - heating of collagen

Coagulated egg white

Milk casein curd

Myofibrillar gel - heating of meat/fish proteins

Bonds that occur in gelation (3 types)

Non-covalent (H bonds) - thermally reversible

Hydrophobic interactions - non thermally reversible

Covalent bonds - not thermally reversible

Steps of conversion of collagen to gelatin (controlled degradation)

Removal of non-collagenous components

Conversion of collagen to gelatin

Recovery of gelatin from solution

Swelling is

expansion due to the uptake of water

Swelling depends on

Type of protein

Protein particle size

pH

Ionic strength

Temp

Dough formation is due to presence and activity of

Gluten

Gluten composed of

Gliadin and glutenin - ratio is important for dough making

Emulsifying properties of proteins depend on

Type of protein

structure

Hydrophobic/philic properties

Temperature

Mechanism of emulsification

Soluble proteins denature and surround oil droplets

Foaming is the dispersion

of gas bubbles in a liquid

Foamable food proteins

Caseins

Lactoglobulin

Egg white proteins

Causes of protein denaturation

Thermal (flexing of protein weakens interactions)

pH

organic solvents

denaturation at interfaces

salt solutions

chemical agents

Protein cross linkers … protein denaturation

reduce