Globular Proteins Hemoglobin & Myoglobin

• Myoglobin - monomer. Single peptide chain and one heme unit– The heme unit contains

an Fe atom (prosthetic group)

• Hemoglobin - tetramer. Four peptide chains and four heme units

Protein Reactions• Hydrolysis: disruption of 1˚ structure by heating in a

strong acid or strong base solution– Reverse of peptide bond formation– Usually, enzyme catalyzed– This is how organisms obtain AA from digested protein

• Denaturation: disruption of 2˚, 3˚ and 4˚ structures

Denaturing Action - Mechanism of Operation:•Heat

•H bonds are broken by increased vibrational energy.(coagulation of egg white albumin on frying.)

•Ultraviolet Radiation•Similar to heat (sunburn): H bonds are broken

•Strong Acids or Bases•salt formation; disruption of hydrogen bonds.•(skin blisters and burns, protein precipitation.)

•Urea SolutionUrea Solution•competition for hydrogen bonds.(precipitation of soluble proteins.)

•Some Organic Solvents(e.g. ethanol & acetone) change ionic groups.(disinfectant action and precipitation of protein.)

•Agitation•shearing of hydrogen bonds.(beating egg white albumin into a meringue.)

Physical and Chemical Denaturing Agents.

Denaturation & Human Hair

Conjugated Proteins

• Proteins that contain prosthetic groups– Glycoproteins– Lipoproteins– Metalloproteins

Collagen from tendons, hooves, etc.denatures and hydrolyzes when

boiled in alkaline conditions and reforms in a chaotic pattern as gelatin

Collagen is a fibrous protein, and is also a glycoprotein.Glucose and Galactose

attach to 5-Hydroxylysine.Carbs cross-link triple helices

into fibril arrangements.Heat ruptures H bonds.

LDL (low density lipoproteins)"particles" found in blood that contain cholesterol. Sphere with single layer, outer boundary made of a phospholipid. Core of sphere holds esterified cholesterol. Inserted into shell are proteins called apoB proteins.

HDL (high density lipoproteins) similar except have apoA protein inserted into phospholipid shell.

Lipoproteins: lipid + protein

esterified cholesterol

Heart Attcack Risk

Structure of an immunoglobulin:a defense glycoprotein

Review – can you…Describe the biological role of various peptide hormones, neurotransmitters, & antioxidantsCompare and contrast simple to conjugated, and fibrous to globular proteins Show products created from protein hydrolysisCompare and contrast physical and chemical denaturing agents, & how they denature proteinsExplain the role and structural differences of glycoproteins and lipoproteins

Enzymes

What is an enzyme?

Enzymes are usually proteins – often globular proteins. They act as catalysts for

biochemical reactions

Can be described in terms of: primary, secondary, tertiary,

and (sometimes) quaternary structures.

Held together by: hydrogen bonds, disulfide bondsdisulfide bonds, salt bridges,

and hydrophobic interactions.

Sound familiar?

Enzyme structure: 2 categories

1. Simple enzymes – composed of just protein2. Conjugated enzyme – protein & non-protein

Protein part = apoenzymeNon-protein part = cofactor (prosthetic group)

Active enzymes usually involve "cofactors:" Sometimes inorganic ions, orSmall organic molecules (a “coenzyme”)

- many vitamins act as coenzymesCofactors are often needed to complete the catalytically active structure of an enzyme.

Biochemical reactions need enzymes to occur. Why? What do enzymes do?

Biochemistry takes place at ~ 37 oC in aqueous environment.

At typical laboratory reaction conditions, to hydrolyze (saponify) fats:

Boil them with concentrated NaOH for hours. Enzymes called lipases do the same thing (hydrolyze fats) at ~ 37 oC without NaOH and it takes minutes.

Catalysts increase the rate of a rxn, but are

not consumed or produced by the rxn.

They do not change Keq of a rxn.

Catalysts do not change the Equilibrium position.

<==>; <-=>; <=->

Catalysts lower the energy barrier

between reactants and products.

Enzyme nomenclature

An enzyme name describes its function:Suffix: –ase e.g. Protease, decarboxylase, ligase

Older names may have the suffix: –ine.g. Pepsin, trypsin, subtilisin, thrombin

Prefix describes enzyme action (function):

Prefix usually names the type of reaction the enzyme is involved in:

Oxidoreductase = Redox reactionsHydrolase = Hydrolysis of bonds

“Substrate” = chemical enzyme acts upon

Specific substrate may also be in the name:Glucose oxidase

Or (less frequently) only the substrate is in the name: Protease, lipase, lactase, etc.