Week 4 Flashcards by Hannah Eastwood

Define lipids

Diverse group of naturally occurring molecules that are soluble in non-polar organic solvents such as chloroform (insoluble or poorly soluble in water)

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Properties of fatty acids

Simple lipids

Building blocks for complex lipids

Source of energy (ATP) for many tissues. (Heart and skeletal muscle ‘prefer’ fatty acids to glucose)

Important for human diet
- essential fatty acids
- unsaturated vs saturated fats
- trans (‘hydrogenated’) fats

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General structure if a fatty acid

Long chain carboxylic acid (n = 10, 12, 14, 16, etc)

Hydrocarbon tail, (hydrophobic; non-polar)

Carboxyl head group (hydrophilic; polar)

Amphipathic = two different chemical characteristics

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Saturated / unsaturated fatty acids

Saturated = C-C bonds

Monounsaturated = One C=C bond in a chain

Polyunsaturated = Multiple C=C bond in a chain

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Fatty acid nomenclature:

Fatty acid have trivial name (group) and systematic names (specific). How are the systematic names worked out?

how many carbons does it have? (Systematic name based in that of parent hydrocarbon)
is it saturated (no double bonds) or unsaturated (with double bonds)
Addition of a suffix: saturated = anoic, monounsaturated = enoic, polyunsaturated = deinoic or trienoic
what position and type of bonds are there

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Essential fatty acid classification:

Omega-3 (alpha-linolenic acid)

Omega-6 (linoleic acid)

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What are essential fatty acids

Unsaturated fatty acids that animals cannot synthesise but are needed to synthesise other fatty acids.

Animals don’t have the enzymes that insert C=C into hydrocarbon chain beyond carbon 9

Plants are therefore a source of (short chain) EFAs in the diet because they possess the enzymes needed for their synthesis
(Fish are also a good source of Omega-3 as they eat micro algae)

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2 types of Complex lipids

Neutral lipids
Polar lipids

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Complex lipids: neural lipids

Include triacylglycerols

Carboxylic acid triesters of glycerol

Main dietary source of fatty acids (in fats and oils)

Storage form of fatty acids
- an efficient energy store
- a major source of biochemical energy

Three fatty acids of any specific triacylglycerol are not necessarily the same. They’re different and varied

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Complex lipids: variation in fatty acids - neural lipids

Properties of triacylglycerol depend on fatty acid composition:

animal fats (larger % of saturated)
veg oil (almost entirely unsaturated)

Bend in tail of unsaturated fatty acid prevents molecules packing tightly together - increased fluidity - lowering melting temperature

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Complex lipids: variation in fatty acids - neural lipids

Animal fats vs veg oils

Animal fats:
- mainly saturated fatty acids
- tightly packed molecules
- high melting point
- solid at room temperature

Veg oil:
- mainly unsaturated fatty acids
- loosely packed molecules
- low melting point
- liquid at room temperature

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Cis and trans double bonds

Naturally occurring unsaturated fatty acids are usually cis (double bond)

Trans doubles bond do not cause a bend - they are more solid at room temperature
They’re produced by partial hydrogenated of vegetable oils implicated in circulatory diseases

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Complex lipids: polar lipids include

Sphingolipids (based on sphingosine)

Glycerolipids:
- Glycosylglycerides (based on glycerol)
- phosphoglycerides (phospholipids)

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Complex lipids: phospholipids and membranes
(Polar lipids)

Have 2 ends with different affinities for water (amphipathic)

In water, phospholipids spontaneously from lipid bilayers

These structures cluster the hydrophobic regions toward the inside and leave the hydrophilic regions exposed to the water environment

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What was the sandwich model? 1935, Davson & Danielli

Phospholipid bilayer between two layers of globular proteins.

Issues: membranes differ in composition and structure
Membrane proteins not water soluble

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Fluid mosaic model? 1972, Singer and Nicolson

Proteins inserted into the membrane sheltering the hydrophobic regions from water

Evidence: freeze-fracture of membrane. Membrane proteins ‘bumps’ into two layers

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Membrane fluidity: movement of phospholipids

Move laterally, very fast

Phospholipid flip-flop (flipping from one side of membrane to other) is more rare but can happen too

Cholesterol role in membrane fluidity

Cholesterol acts as a temperature buffer and barriers to the free movement of phospholipids, enabling stabilisation of the membrane

High temp reduce movement so less fluid
Low temp hinder packing so maintain fluidity

Membrane fluidity: movement of proteins

Frye and Edidin study ?

Move laterally

Frye and Edidin 1970 - fusion studies in mouse and human cells labelled with different markers. If proteins fixed, markers would stay in discrete regions. But if move freely, protein markers would be distributed. - saw even distribution of protein markers showing proteins move freely within the membrane

Three main importance of bio-membranes

Separate the cell contents from its surroundings (divides cell into compartments eg organelles)

Maintain optimal different biochemical environments between inside and outside of cell/organelles

Selectively permeable (ability of the cell to discriminate in its chemical exchanges with the environment. fundamental to like)

Models of a membrane: 1915 vs 1925

1915:
Red blood cell (erythrocyte) membranes were first isolated and analyzed

1925:
Gorter and Grendel
Membranes described as a bilayer of phospholipids

Fluidity of membranes: Fatty acid saturation

Fluid: unsaturated hydrocarbon tails (kinked) prevent packing, enhancing membrane fluidity.

Viscous: Saturated hydrocarbon tails pack together, increasing membrane viscosity

(Kinks keep phospholipids from packing together, enhancing membrane fluidity

Hop diffusion modification of fluid mosaic model: 2001, Akihiro Kusumi

Proteins don’t just move randomly and freely, they’re corralled by fences in the cell’s actin cytoskeleton, but can ‘hop’ into another area

Mosaic nature of membranes (current, modified membrane model): what three main groups are proteins classified into?

Integral proteins (transmembrane protein)
Peripheral proteins (loosely bound via other proteins)
Lipid-anchored proteins (covalently attached to lipids eg GPI-anchored)

Membrane proteins: protein structure determines what?

Protein structure (hydrophilic and hydrophobic domains) determine how a protein associates with a membrane

Membrane carbohydrates structure

Short, branched oligosaccharide chains of,<15 sugar units. Highly diverse (between species, individuals, cells) Act as identity tags (eg blood groups are due to variation in carbohydrates on the surface of RBC) Most covalently bound to proteins; glycoproteins Some covalently bound to lipids; glycolipids

Membrane asymmetry: Membranes have distinct inside/outside faces that differ in what?

Lipid composition Peripheral proteins attached Carbohydrates attached (Integral proteins are asymmetrical with a defined orientation)

Membrane function: major roles of the membrane?

Compartmentalisation - Cells, organelles - Spatial organisation of biochemical reactions To provide a selectively permeable barrier Cell-to-cell recognition/communication

Membrane function: selectively permeable barrier

Rate of diffusion across the bilayer depends on: - Size of molecule - Relative solubility in oil (polarity/charge) Highly permeable to small non-polar molecules and small uncharged polar molecules depending on size (small > large) Highly impermeable to charged molecules (ions) and large polar molecules

Membrane function: transport proteins How are ions and large molecules like glucose and amino acids able to move efficiently across biological membranes?

Proteins with a hydrophilic channel allowing ions or molecules to diffuse passively from one side to the other Proteins that bind and transport molecules in an energy requiring process (Transport proteins are usually quite specific for the substance transported)

Membrane function: cell-cell recognition/communication

Some glycoproteins act as identification tags that are recognised by receptor proteins in other cells The ability to distinguish one type of cell from another is crucial to for example: - During embryogenesis for the sorting of cells into tissues and organs - As the basis for recognition and rejection of foreign cells by the immune system

Membrane function: what are further functions of membrane proteins?

Enzyme activity (sometimes organised in 'teams' to carry out sequential steps in a metabolic pathway) Signal transduction (receptors for chemical messengers (signal molecules)) Intercellular joining ('gap junctions' or 'tight junctions') Attachment to the cytoskeleton and extracellular matrix - EMC ( Coordination of extracellular processes with intracellular processes. Maintains cell shape. Stabilises location of the protein)

In monosaccharide bonding what bond forms between anomeric carbon of one sugar and the hydroxyl group of another?

Glycosidic bond Condensation reaction producing water

What is a reducing sugar?

Sugars containing a free aldehyde or ketone group In solution, the open-chain forms of aldose and ketoses can reduce indicators such as Cu2+ to Cu+

Carbohydrates: structural diversity.

Carbohydrates can achieve great structural diversity because of the many types of monosaccharides and linkage possibilities.

What do glycosyltransferases join together?

Sugars

Do carbohydrates have a direction?

Yes! The reducing end: The anomeric of Glc is not linked so the sugar cannot mutarotate eg convert through the open-chain form, therefore is a reducing sugar Non-reducing end: Anomeric of Gal is linked so the sugar cannot mutarotate, eg convert through the open-chain form, therefore is non-reducing.

What is glycolysis?

A series of reactions in the cytosol of the cell that: - Convert glucose into 2x three carbon molecules (Pyruvate) - In doing so some of the chemical energy stored in glucose is released as ATP. For each molecule of glucose converted, there is a net total of 2 molecules of ATP.

Where is pyruvate transported to in the presence of molecular oxygen?

Into the mitochondrion Entering citric acid cycle / Krebs cycle / Tricarboxylic acid cycle.

Before entering the citric acid cycle pyruvate is converted into what?

Acetyl-Coenzyme A This takes place in the matrix of the mitochondria

Citric acid cycle (summary)

- Represented as 8 enzymatic reactions: - Acetyl-CoA joins cycle by binding to Oxaloacetate forming citrate - Following 7 steps complete the decomposition of citrate into oxaloacetate and therefore a cycle is formed. - NADH and FADH2 are fed into the electron transport chains and this is where majority of ATP is produced